Signal transduction

Signal transduction

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Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response. There are two stages in this process:
  1. A signaling molecule activates a specific receptor on the cell membrane
  2. Causing a second messenger to continue the signal into the cell and elicit a physiological response.

In either step, the signal can be amplified. Thus, one signalling molecule can cause many responses.

History



In 1970, Martin Rodbell
Martin Rodbell
Martin Rodbell was an American biochemist and molecular endocrinologist who is best known for his discovery of G-proteins. He shared the 1994 Nobel Prize in Physiology or Medicine with Alfred G...

 examined the effects of glucagon
Glucagon
Glucagon, a hormone secreted by the pancreas, raises blood glucose levels. Its effect is opposite that of insulin, which lowers blood glucose levels. The pancreas releases glucagon when blood sugar levels fall too low. Glucagon causes the liver to convert stored glycogen into glucose, which is...

 on a rat's liver cell membrane receptor. He noted that guanosine triphosphate
Guanosine triphosphate
Guanosine-5'-triphosphate is a purine nucleoside triphosphate. It can act as a substrate for the synthesis of RNA during the transcription process...

 disassociated glucagon from this receptor and stimulated the G-protein, which strongly influenced the cell's metabolism. Thus he deduced that the G-protein was a transducer that accepted glucagon molecules and affected the cell. For this he shared the 1994 Nobel Prize in Physiology or Medicine
Nobel Prize in Physiology or Medicine
The Nobel Prize in Physiology or Medicine administered by the Nobel Foundation, is awarded once a year for outstanding discoveries in the field of life science and medicine. It is one of five Nobel Prizes established in 1895 by Swedish chemist Alfred Nobel, the inventor of dynamite, in his will...

 with Alfred G. Gilman
Alfred G. Gilman
Alfred Goodman Gilman is an American pharmacologist and biochemist. He shared the 1994 Nobel Prize in Physiology or Medicine with Martin Rodbell for their discoveries regarding G-proteins....

. The current understanding of signal transduction processes reflects contributions made by Rodbell and many other research groups.

The earliest scientific paper
Academic publishing
Academic publishing describes the subfield of publishing which distributes academic research and scholarship. Most academic work is published in journal article, book or thesis form. The part of academic written output that is not formally published but merely printed up or posted is often called...

 recorded in the MEDLINE
MEDLINE
MEDLINE is a bibliographic database of life sciences and biomedical information. It includes bibliographic information for articles from academic journals covering medicine, nursing, pharmacy, dentistry, veterinary medicine, and health care...

 database as containing the specific term signal transduction was published in 1972. Some articles published before 1977 used the term signal transmission or sensory transduction for signal transduction: a total of 48,377 scientific papers related to signal transduction were published in 1977, of which 11,211 were reviews of other papers
Review journal
A review journal in academic publishing is a periodical or series that is devoted to the publication of review articles that summarize the progress in some particular area or topic during a preceding period.-Types:Review journals can be divided by...

. That year the actual term signal transduction was included in abstracts
Abstract (summary)
An abstract is a brief summary of a research article, thesis, review, conference proceeding or any in-depth analysis of a particular subject or discipline, and is often used to help the reader quickly ascertain the paper's purpose. When used, an abstract always appears at the beginning of a...

 until in 1979 it appeared in a paper's title. One source attributes the widespread use of this term to a 1980 review article by Rodbell: research papers directly addressing signal transduction processes began to appear in large numbers in the late 1980s and early 1990s.

Signaling molecules


Signal transduction involves the binding of extracellular signalling molecules and ligand
Ligand (biochemistry)
In biochemistry and pharmacology, a ligand is a substance that forms a complex with a biomolecule to serve a biological purpose. In a narrower sense, it is a signal triggering molecule, binding to a site on a target protein.The binding occurs by intermolecular forces, such as ionic bonds, hydrogen...

s to cell-surface receptors that trigger events inside the cell. Intracellular signaling cascades can be started through cell-substratum interactions; examples are the integrin
Integrin
Integrins are receptors that mediate attachment between a cell and the tissues surrounding it, which may be other cells or the ECM. They also play a role in cell signaling and thereby regulate cellular shape, motility, and the cell cycle....

 that binds ligands in the extracellular matrix
Extracellular matrix
In biology, the extracellular matrix is the extracellular part of animal tissue that usually provides structural support to the animal cells in addition to performing various other important functions. The extracellular matrix is the defining feature of connective tissue in animals.Extracellular...

 and steroid
Steroid
A steroid is a type of organic compound that contains a characteristic arrangement of four cycloalkane rings that are joined to each other. Examples of steroids include the dietary fat cholesterol, the sex hormones estradiol and testosterone, and the anti-inflammatory drug dexamethasone.The core...

s. Most steroid hormones have receptors within the cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...

 and act by stimulating the binding of their receptors to the promoter region of steroid-responsive genes. Examples of signaling molecules include the hormone melatonin
Melatonin
Melatonin , also known chemically as N-acetyl-5-methoxytryptamine, is a naturally occurring compound found in animals, plants, and microbes...

, the neurotransmitter acetylcholine
Acetylcholine
The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...

 and the cytokine
Cytokine
Cytokines are small cell-signaling protein molecules that are secreted by the glial cells of the nervous system and by numerous cells of the immune system and are a category of signaling molecules used extensively in intercellular communication...

 interferon γ
Interferon-gamma
Interferon-gamma is a dimerized soluble cytokine that is the only member of the type II class of interferons. This interferon was originally called macrophage-activating factor, a term now used to describe a larger family of proteins to which IFN-γ belongs...

.

The classifications of signalling molecules do not take into account the molecular nature of each class member; neurotransmitters range in size from small molecules such as dopamine
Dopamine
Dopamine is a catecholamine neurotransmitter present in a wide variety of animals, including both vertebrates and invertebrates. In the brain, this substituted phenethylamine functions as a neurotransmitter, activating the five known types of dopamine receptors—D1, D2, D3, D4, and D5—and their...

 to neuropeptides such as endorphins. Some molecules may fit into more than one class; for example, epinephrine
Epinephrine
Epinephrine is a hormone and a neurotransmitter. It increases heart rate, constricts blood vessels, dilates air passages and participates in the fight-or-flight response of the sympathetic nervous system. In chemical terms, adrenaline is one of a group of monoamines called the catecholamines...

 is a neurotransmitter when secreted by the central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

 and a hormone when secreted by the adrenal medulla
Adrenal medulla
The adrenal medulla is part of the adrenal gland. It is located at the center of the gland, being surrounded by the adrenal cortex. It is the innermost part of the adrenal gland, consisting of cells that secrete epinephrine , norepinephrine , and a small amount of dopamine in response to...

.

Environmental stimuli


With single-celled organisms, the variety of signal transduction processes influence its reaction to its environment. With multicellular organisms, numerous processes are required for coordinating individual cells to support the organism as a whole; the complexity of these processes tend to increase with the complexity of the organism. Sensing
Sense
Senses are physiological capacities of organisms that provide inputs for perception. The senses and their operation, classification, and theory are overlapping topics studied by a variety of fields, most notably neuroscience, cognitive psychology , and philosophy of perception...

 of environments at the cellular level relies on signal transduction; many disease processes, such as diabetes and heart disease
Atherosclerosis
Atherosclerosis is a condition in which an artery wall thickens as a result of the accumulation of fatty materials such as cholesterol...

 arise from defects in these pathways, highlighting the importance of this process in biology and medicine.

Various environmental stimuli exist that initiate signal transmission processes in multicellular organisms; examples include photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

s hitting cells in the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

 of the eye, and odorants binding to odorant receptors
Olfactory receptor neuron
An olfactory receptor neuron , also called an olfactory sensory neuron , is a transduction cell within the olfactory system. J. Rospars, Dendritic integration in olfactory sensory neurons: a steady-state analysis of how the neuron structure and neuron environment influence the coding of odor...

 in the nasal epithelium
Olfactory epithelium
The olfactory epithelium is a specialized epithelial tissue inside the nasal cavity that is involved in smell. In humans, it measures about 1 square centimetre and lies on the roof of the nasal cavity about 7 cm above and behind the nostrils...

. Certain microbial molecules, such as viral nucleotide
Nucleotide
Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...

s and protein antigen
Antigen
An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as...

s, can elicit an immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

 response against invading pathogen
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...

s mediated by signal transduction processes. This may occur independent of signal transduction stimulation by other molecules, as is the case for the toll-like receptor
Toll-like receptor
Toll-like receptors are a class of proteins that play a key role in the innate immune system. They are single, membrane-spanning, non-catalytic receptors that recognize structurally conserved molecules derived from microbes...

. It may occur with help from stimulatory molecules located at the cell surface of other cells, as with T-cell receptor signaling. Single-celled organisms may respond to environmental stimuli through the activation of signal transduction pathways. For example, slime molds
Dictyostelid
The dictyostelids are a group of cellular slime molds, or social amoebae.-Slug behavior:When food is readily available they are individual amoebae, which feed and divide normally...

 secrete cyclic adenosine monophosphate
Cyclic adenosine monophosphate
Cyclic adenosine monophosphate is a second messenger important in many biological processes...

 upon starvation, stimulating individual cells in the immediate environment to aggregate, and yeast
Yeast
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...

 cells use mating factor
Mating of yeast
The yeast Saccharomyces cerevisiae is a simple single celled eukaryote with both a diploid and haploid mode of existence. The mating of yeast only occurs between haploids, which can be either the a or α mating type and thus display simple sexual differentiation...

s to determine the mating types of other cells and to participate in sexual reproduction.

Receptors


Receptors can be roughly divided into two major classes: intracellular
Intracellular
Not to be confused with intercellular, meaning "between cells".In cell biology, molecular biology and related fields, the word intracellular means "inside the cell".It is used in contrast to extracellular...

 receptors and extracellular
Extracellular
In cell biology, molecular biology and related fields, the word extracellular means "outside the cell". This space is usually taken to be outside the plasma membranes, and occupied by fluid...

 receptors.

Extracellular


Extracellular receptors are integral transmembrane protein
Transmembrane protein
A transmembrane protein is a protein that goes from one side of a membrane through to the other side of the membrane. Many TPs function as gateways or "loading docks" to deny or permit the transport of specific substances across the biological membrane, to get into the cell, or out of the cell as...

s and make up most receptors. They span the plasma membrane of the cell, with one part of the receptor on the outside of the cell and the other on the inside. Signal transduction occurs as a result of a ligand binding to the outside; the molecule does not pass through the membrane. This binding stimulates a series of events inside the cell; different types of receptor stimulate different responses and receptors typically respond to only the binding of a specific ligand. Upon binding, the ligand induces a change in the conformation of the inside part of the receptor. These result in either the activation of an enzyme in the receptor or the exposure of a binding site for other intracellular signaling proteins within the cell, eventually propagating the signal through the cytoplasm.

In eukaryotic
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...

 cells, most intracellular proteins activated by a ligand/receptor interaction possess an enzymatic activity; examples include tyrosine kinase
Tyrosine kinase
A tyrosine kinase is an enzyme that can transfer a phosphate group from ATP to a protein in a cell. It functions as an "on" or "off" switch in many cellular functions....

 and phosphatases. Some of them create second messengers such as cyclic AMP and IP3
Inositol triphosphate
Inositol trisphosphate or inositol 1,4,5-trisphosphate , together with diacylglycerol , is a secondary messenger molecule used in signal transduction and lipid signaling in biological cells. While DAG stays inside the membrane, IP3 is soluble and diffuses through the cell...

, the latter controlling the release of intracellular calcium stores into the cytoplasm. Other activated proteins interact with adapter proteins that facilitate signalling protein interactions and coordination of signalling complexes necessary to respond to a particular stimulus. Enzymes and adapter proteins are both responsive to various second messenger molecules.

Many adapter proteins and enzymes activated as part of signal transduction possess specialized protein domains that bind to specific secondary messenger molecules. For example, calcium ions bind to the EF hand
EF hand
The EF hand is a helix-loop-helix structural domain found in a large family of calcium-binding proteins. The EF-hand motif contains a helix-loop-helix topology, much like the spread thumb and forefinger of the human hand, in which the Ca2+ ions are coordinated by ligands within the loop...

 domains of calmodulin
Calmodulin
Calmodulin is a calcium-binding protein expressed in all eukaryotic cells...

, allowing it to bind and activate calmodulin-dependent kinase. PIP3 and other phosphoinositides do the same thing to the Pleckstrin homology domain
Pleckstrin homology domain
Pleckstrin homology domain is a protein domain of approximately 120 amino acids that occurs in a wide range of proteins involved in intracellular signaling or as constituents of the cytoskeleton....

s of proteins such as the kinase protein AKT
AKT
Akt, also known as Protein Kinase B , is a serine/threonine protein kinase that plays a key role in multiple cellular processes such as glucose metabolism, cell proliferation, apoptosis, transcription and cell migration.-Family members:...

.

G protein-coupled


G protein-coupled receptors (GPCRs) are a family of integral transmembrane proteins that possess seven transmembrane domains and are linked to a heterotrimeric G protein
G protein
G proteins are a family of proteins involved in transmitting chemical signals outside the cell, and causing changes inside the cell. They communicate signals from many hormones, neurotransmitters, and other signaling factors. G protein-coupled receptors are transmembrane receptors...

. Many receptors are in this family, including adrenergic receptor
Adrenergic receptor
The adrenergic receptors are a class of metabotropic G protein-coupled receptors that are targets of the catecholamines, especially noradrenaline and adrenaline ....

s and chemokine receptor
Chemokine receptor
Chemokine receptors are cytokine receptors found on the surface of certain cells, which interact with a type of cytokine called a chemokine. There have been 19 distinct chemokine receptors described in mammals...

s.

Signal transduction by a GPCR begins with an inactive G protein coupled to the receptor; it exists as a heterotrimer consisting of Gα, Gβ, and Gγ. Once the GPCR recognizes a ligand, the conformation of the receptor changes to activate the G protein, causing Gα to bind a molecule of GTP and dissociate from the other two G-protein subunits. The dissociation exposes sites on the subunits that can interact with other molecules. The activated G protein subunits detach from the receptor and initiate signalling from many downstream effector proteins such as phospholipase
Phospholipase
A phospholipase is an enzyme that hydrolyzes phospholipids into fatty acids and other lipophilic substances. There are four major classes, termed A, B, C and D, distinguished by the type of reaction which they catalyze:*Phospholipase A...

s and ion channels, the latter permitting the release of second messenger molecules. The total strength of signal amplification by a GPCR is determined by the lifetimes of the ligand-receptor complex and receptor-effector protein complex and the deactivation time of the activated receptor and effectors through intrinsic enzymatic activity.

A study was conducted where a point mutation
Point mutation
A point mutation, or single base substitution, is a type of mutation that causes the replacement of a single base nucleotide with another nucleotide of the genetic material, DNA or RNA. Often the term point mutation also includes insertions or deletions of a single base pair...

 was inserted into the gene encoding the chemokine
Chemokine
Chemokines are a family of small cytokines, or proteins secreted by cells. Their name is derived from their ability to induce directed chemotaxis in nearby responsive cells; they are chemotactic cytokines...

 receptor CXCR2; mutated cells underwent a malignant transformation
Malignant transformation
Malignant transformation is the process by which cells acquire the properties of cancer. This may occur as a primary process in normal tissue, or secondarily as malignant degeneration of a previously existing benign tumor....

 due to the expression
Gene expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...

 of CXCR2 in an active conformation despite the absence of chemokine-binding. This meant that chemokine receptors participate in cancer development.

Tyrosine and histidine kinase


Receptor tyrosine kinase
Receptor tyrosine kinase
Receptor tyrosine kinases s are the high-affinity cell surface receptors for many polypeptide growth factors, cytokines, and hormones. Of the 90 unique tyrosine kinase genes identified in the human genome, 58 encode receptor tyrosine kinase proteins....

s (RTKs) are transmembrane proteins with an intracellular kinase
Kinase
In chemistry and biochemistry, a kinase is a type of enzyme that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific substrates, a process referred to as phosphorylation. Kinases are part of the larger family of phosphotransferases...

 domain and an extracellular domain that binds ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...

s; examples include growth factor
Growth factor
A growth factor is a naturally occurring substance capable of stimulating cellular growth, proliferation and cellular differentiation. Usually it is a protein or a steroid hormone. Growth factors are important for regulating a variety of cellular processes....

 receptors such as the insulin receptor
Insulin
Insulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....

. To perform signal transduction, RTKs need to form dimer
Protein dimer
In biochemistry, a dimer is a macromolecular complex formed by two, usually non-covalently bound, macromolecules like proteins or nucleic acids...

s in the plasma membrane; the dimer is stabilized by ligands binding to the receptor. The interaction between the cytoplasmic domains stimulates the autophosphorylation
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....

 of tyrosine
Tyrosine
Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...

s within the domains of the RTKs, causing conformational changes. The receptors' kinase domains are subsequently activated, initiating phosphorylation
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....

 signalling cascades of downstream cytoplasmic molecules that facilitate various cellular processes such as cell differentiation and metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

.

As is the case with GPCRs, proteins that bind GTP play a major role in signal transduction from the activated RTK into the cell. In this case, the G proteins are members of the Ras
Ras superfamily
The Ras superfamily is a protein superfamily of small GTPases, which are all related, to a degree, to the Ras protein subfamily .There are more than a hundred proteins in the Ras superfamily...

, Rho
Rho family of GTPases
The Rho family of GTPases is a family of small signaling G protein , and is a subfamily of the Ras superfamily. The members of the Rho GTPase family have been shown to regulate many aspects of intracellular actin dynamics, and are found in all eukaryotic organisms including yeasts and some plants...

, and Raf families, referred to collectively as small G proteins. They act as molecular switches usually tethered to membranes by isoprenyl groups linked to their carboxyl ends. Upon activation, they assign proteins to specific membrane subdomains where they participate in signaling. Activated RTKs in turn activate small G proteins that activate guanine nucleotide exchange factor
Guanine nucleotide exchange factor
Guanine nucleotide exchange factors activate monomeric GTPases by stimulating the release of guanosine diphosphate to allow binding of guanosine triphosphate . A variety of unrelated structural domains have been shown to exhibit guanine nucleotide exchange activity...

s such as SOS1
SOS1
Son of sevenless homolog 1 is a protein that in humans is encoded by the SOS1 gene.Recent studies also show that mutations in Sos1 can cause Noonan syndrome and hereditary gingival fibromatosis type 1. Noonan syndrome has also been shown to be caused by mutations in KRAS and PTPN11 genes...

. Once activated, these exchange factors can activate more small G proteins, thus amplifying the receptor's initial signal. The mutation of certain RTK genes, as with that of GPCRs, can result in the expression
Gene expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...

 of receptors that exist in a constitutively-activate state; such mutated genes may act as oncogenes.

Histidine-specific protein kinases
Histidine kinase
Histidine Kinases are multifunctional, typically transmembrane, proteins of the transferase class that play a role in signal transduction across the cellular membrane. The vast majority of HKs are homodimers that exhibit autokinase, phosphotransfer, and phosphatase activity. HKs can act as...

 are structurally distinct from other protein kinases and are found in prokaryotes, fungi, and plants as part of a two-component signal transduction mechanism: a phosphate group from ATP is first added to a histidine residue within the kinase, then transferred to an aspartate residue on a receiver domain on a different protein or the kinase itself, thus activating the aspartate residue.

Integrin



Integrins are produced by a wide variety of cells; they play a role in cell attachment to other cells and the extracellular matrix
Extracellular matrix
In biology, the extracellular matrix is the extracellular part of animal tissue that usually provides structural support to the animal cells in addition to performing various other important functions. The extracellular matrix is the defining feature of connective tissue in animals.Extracellular...

 and in the transduction of signals from extracellular matrix components such as fibronectin
Fibronectin
Fibronectin is a high-molecular weight glycoprotein of the extracellular matrix that binds to membrane-spanning receptor proteins called integrins. In addition to integrins, fibronectin also binds extracellular matrix components such as collagen, fibrin and heparan sulfate proteoglycans...

 and collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...

. Ligand binding to the extracellular domain of integrins changes the protein's conformation, clustering it at the cell membrane to initiate signal transduction. Integrins lack kinase activity; hence integrin-mediated signal transduction is achieved through a variety of intracellular protein kinases and adaptor molecules, the main coordinator being integrin-linked kinase
Integrin-linked kinase
Integrin-linked kinase is a 59kDa protein originally identified while conducting a yeast-two hybrid screen with integrin β1 as the bait protein...

. As shown in the picture to the right, cooperative integrin-RTK signalling determines the timing of cellular survival, apoptosis
Apoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...

, proliferation
Cell growth
The term cell growth is used in the contexts of cell development and cell division . When used in the context of cell division, it refers to growth of cell populations, where one cell grows and divides to produce two "daughter cells"...

, and differentiation
Cellular differentiation
In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a simple zygote to a complex system of...

.

Important differences exist between integrin signalling in circulating blood cells and non-circulating cells such as epithelial cells; integrins of circulating cells are normally inactive. For example, cell membrane integrins on circulating leukocytes are maintained in an inactive state to avoid epithelial cell attachment; they are only activated in response to stimuli such as those received at the site of an inflammatory response
Inflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...

. In a similar manner, integrins at the cell membrane of circulating platelets are normally kept inactive to avoid thrombosis
Thrombosis
Thrombosis is the formation of a blood clot inside a blood vessel, obstructing the flow of blood through the circulatory system. When a blood vessel is injured, the body uses platelets and fibrin to form a blood clot to prevent blood loss...

. Epithelial cells (which are non-circulating) normally have active integrins at their cell membrane, helping maintain their stable adhesion to underlying stromal cells that provide signals to maintain normal functioning.

Toll gate


When activated, toll-like receptors (TLRs) take adapter molecules within the cytoplasm of cells in order to propagate a signal. Four adaptor molecules are known to be involved in signaling, which are Myd88
Myd88
Myeloid differentiation primary response gene is a protein that, in humans, is encoded by the MYD88 gene.-Function:In mice, MyD88 is a universal adapter protein as it is used by all TLRs to activate the transcription factor NF-κB. Mal is necessary to recruit Myd88 to TLR 2 and TLR 4, and MyD88...

, TIRAP
TIRAP
TIRAP is an adapter molecule associated with toll-like receptors....

, TRIF
Trif
TIR-domain-containing adapter-inducing interferon-β is an adapter in responding to activation of toll-like receptors . It mediates the rather delayed cascade of two TLR-associated signaling cascades, where the other one is dependent upon a MyD88 adapter.Toll-like receptors recognize specific...

, and TRAM. These adapters activate other intracellular molecules such as IRAK1
IRAK1
Interleukin-1 receptor-associated kinase 1 is an enzyme that in humans is encoded by the IRAK1 gene.-Interactions:IRAK1 has been shown to interact with Myd88, IKBKG, IKK2, TRAF6, CHUK and Ubiquitin C.-Further reading:...

, IRAK4, TBK1, and IKKi
Ikki
The term Ikki can refer to:*In Japanese history, leagues of samurai, farmers, and clergy who engaged in common defense against shogunal forces and greater lords, initiating large and destructive agrarian uprisings. The uprisings were also called ikki...

 that amplify the signal, eventually leading to the induction or suppression of genes that cause certain responses. Thousands of genes are activated by TLR signaling, implying that this method constitutes an important gateway for gene modulation.

Ligand-gated ion channel


A ligand-gated ion channel, upon binding with a ligand, changes conformation to open a channel in the cell membrane through which ions relaying signals can pass. An example of this mechanism is found in the receiving cell of a neural synapse
Synapse
In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...

. The influx of ions that occurs in response to these channels opening induces action potentials, such as those that travel along nerves, by depolarizing the membrane of post-synaptic cells, resulting in the opening of voltage-gated ion channels.

An example of an ion allowed into the cell during a ligand-gated ion channel opening is Ca2+; it acts as a second messenger initiating signal transduction cascades and altering the physiology of the responding cell. This results in amplification of the synapse response between synaptic cells by remodelling the dendritic spines involved in the synapse.

Intracellular


Intracellular receptors, such as nuclear receptor
Nuclear receptor
In the field of molecular biology, nuclear receptors are a class of proteins found within cells that are responsible for sensing steroid and thyroid hormones and certain other molecules...

s and cytoplasmic receptors
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...

, are soluble proteins localized within their respective areas. The typical ligands for nuclear receptors are lipophilic
Lipophilic
Lipophilicity, , refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene. These non-polar solvents are themselves lipophilic — the axiom that like dissolves like generally holds true...

 hormones like the steroid
Steroid
A steroid is a type of organic compound that contains a characteristic arrangement of four cycloalkane rings that are joined to each other. Examples of steroids include the dietary fat cholesterol, the sex hormones estradiol and testosterone, and the anti-inflammatory drug dexamethasone.The core...

 hormones testosterone
Testosterone
Testosterone is a steroid hormone from the androgen group and is found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is primarily secreted in the testes of males and the ovaries of females, although small amounts are also secreted by the adrenal glands...

 and progesterone
Progesterone
Progesterone also known as P4 is a C-21 steroid hormone involved in the female menstrual cycle, pregnancy and embryogenesis of humans and other species...

 and derivatives of vitamins A and D. To initiate signal transduction, the ligand must pass through the plasma membrane by passive diffusion. On binding with the receptor, the ligands pass through the nuclear membrane into the nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...

, enabling gene transcription and protein production.

Activated nuclear receptors attach to the DNA at receptor-specific hormone-responsive element (HRE) sequences, located in the promoter region of the genes activated by the hormone-receptor complex. Due to them enabling gene transcription, they are alternatively called inductors of gene expression
Gene expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...

. Activation of gene transcription is slower than signals directly affecting existing proteins; therefore, the effects of hormones that use nucleic receptors are long-term.

Signal transduction via these receptors involves little proteins, but the details of gene regulation by this method are not well understood. Nucleic receptors have DNA-binding domains containing zinc finger
Zinc finger
Zinc fingers are small protein structural motifs that can coordinate one or more zinc ions to help stabilize their folds. They can be classified into several different structural families and typically function as interaction modules that bind DNA, RNA, proteins, or small molecules...

s and a ligand-binding domain; the zinc fingers stabilize DNA binding by holding its phosphate backbone. DNA sequences that match the receptor are usually hexameric repeats of any kind; the sequences are similar but their orientation and distance differentiate them. The ligand-binding domain is additionally responsible for dimerization
Protein dimer
In biochemistry, a dimer is a macromolecular complex formed by two, usually non-covalently bound, macromolecules like proteins or nucleic acids...

 of nucleic receptors prior to binding and providing structures for transactivation
Transactivation
In molecular biology and genetics, transactivation is an increased rate of gene expression triggered either by biological processes or by artificial means.- Natural transactivation :...

 used for communication with the translational apparatus.

Steroid receptors are a subclass of nuclear receptors located primarily within the cytosol; in the absence of steroids, they cling together in an aporeceptor complex containing chaperone or heatshock proteins (HSPs). The HSPs are necessary to activate the receptor by assisting the protein to fold
Protein folding
Protein folding is the process by which a protein structure assumes its functional shape or conformation. It is the physical process by which a polypeptide folds into its characteristic and functional three-dimensional structure from random coil....

 in a way such that the signal sequence
Signal sequence
Signal sequence can refer to:*Protein targeting*Signal peptide*DNA uptake signal sequence...

 enabling its passage into the nucleus is accessible. Steroid receptors, on the other hand, may be repressive on gene expression when their transactivation domain is hidden; activity can be enhanced by phosphorylation of serine
Serine
Serine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:...

 residues at their N-terminal as a result of another signal transduction pathway, a process called crosstalk
Crosstalk (biology)
Biological crosstalk refers to instances in which one or more components of a signal transduction pathway affect a different pathway. This can be achieved through a number of ways with the most common form being crosstalk between proteins of signaling cascades. In these signal transduction...

.

Retinoic acid receptor
Retinoic acid receptor
The retinoic acid receptor is a type of nuclear receptor that is activated by both all-trans retinoic acid and 9-cis retinoic acid. There are three retinoic acid receptors , RAR-alpha, RAR-beta, and RAR-gamma, encoded by the , , genes, respectively...

s are another subset of nuclear receptors. They can be activated by an endocrine-synthesized ligand that entered the cell by diffusion, a ligand synthesised from a precursor
Protein precursor
A protein precursor, also called a pro-protein or pro-peptide, is an inactive protein that can be turned into an active form by posttranslational modification. The name of the precursor for a protein is often prefixed by pro...

 like retinol
Retinol
Retinol is one of the animal forms of vitamin A. It is a diterpenoid and an alcohol. It is convertible to other forms of vitamin A, and the retinyl ester derivative of the alcohol serves as the storage form of the vitamin in animals....

 brought to the cell through the bloodstream or a completely intracellularly synthesised ligand like prostaglandin
Prostaglandin
A prostaglandin is any member of a group of lipid compounds that are derived enzymatically from fatty acids and have important functions in the animal body. Every prostaglandin contains 20 carbon atoms, including a 5-carbon ring....

. These receptors are located in the nucleus and are not accompanied by HSPs; they repress their gene by binding to their specific DNA sequence when no ligand binds to them and vice versa.

Certain intracellular receptors of the immune system are cytoplasmic receptors; recently identified NOD-like receptors
Pattern recognition receptor
Pattern recognition receptors are a primitive part of the immune system. They are proteins expressed by cells of the innate immune system to identify pathogen-associated molecular patterns , which are associated with microbial pathogens or cellular stress, as well as damage-associated molecular...

 (NLRs) reside in the cytoplasm of some eukaryotic cells and interact with ligands using a leucine-rich repeat
Leucine-rich repeat
A leucine-rich repeat is a protein structural motif that forms an α/β horseshoe fold. It is composed of repeating 20–30 amino acid stretches that are unusually rich in the hydrophobic amino acid leucine...

 (LRR) motif similar to TLRs. Some of these molecules like NOD2 interact with RIP2 kinase that activates NF-κB signaling, whereas others like NALP3 interact with inflammatory caspase
Caspase
Caspases, or cysteine-aspartic proteases or cysteine-dependent aspartate-directed proteases are a family of cysteine proteases that play essential roles in apoptosis , necrosis, and inflammation....

s and initiate processing of particular cytokine
Cytokine
Cytokines are small cell-signaling protein molecules that are secreted by the glial cells of the nervous system and by numerous cells of the immune system and are a category of signaling molecules used extensively in intercellular communication...

s like interleukin-1β.

Second messengers


Intracellular signal transduction is carried out by second messenger
Second messenger system
Second messengers are molecules that relay signals from receptors on the cell surface to target molecules inside the cell, in the cytoplasm or nucleus. They relay the signals of hormones like epinephrine , growth factors, and others, and cause some kind of change in the activity of the cell...

 molecules.

Calcium


The release of calcium ions from the endoplasmic reticulum
Endoplasmic reticulum
The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...

 into the cytosol
Cytosol
The cytosol or intracellular fluid is the liquid found inside cells, that is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into compartments....

 results in its binding to signaling proteins that are then activated; it is then sequestered in the smooth endoplasmic reticulum and the mitochondria. Two combined receptor/ion channel proteins control the transport of calcium: the InsP3-receptor that transports calcium upon interaction with inositol triphosphate
Inositol triphosphate
Inositol trisphosphate or inositol 1,4,5-trisphosphate , together with diacylglycerol , is a secondary messenger molecule used in signal transduction and lipid signaling in biological cells. While DAG stays inside the membrane, IP3 is soluble and diffuses through the cell...

 on its cytosolic side and the ryanodine receptor
Ryanodine receptor
Ryanodine receptors form a class of intracellular calcium channels in various forms of excitable animal tissue like muscles and neurons...

 named after the alkaloid
Alkaloid
Alkaloids are a group of naturally occurring chemical compounds that contain mostly basic nitrogen atoms. This group also includes some related compounds with neutral and even weakly acidic properties. Also some synthetic compounds of similar structure are attributed to alkaloids...

 ryanodine
Ryanodine
Ryanodine is a poisonous alkaloid found in the South American plant Ryania speciosa . It was originally used as an insecticide....

, similar to the InsP3 receptor but having a feedback mechanism that releases more calcium upon binding with it. The nature of calcium in the cytosol means that it is active for only a very short time, meaning its free state concentration is very low and is mostly bound to organelle molecules like calreticulin
Calreticulin
Calreticulin also known as calregulin, CRP55, CaBP3, calsequestrin-like protein, and endoplasmic reticulum resident protein 60 is a protein that in humans is encoded by the CALR gene....

 when inactive.

Calcium is used in many processes including muscle contraction, neurotransmitter release from nerve endings and cell migration
Cell migration
Cell migration is a central process in the development and maintenance of multicellular organisms. Tissue formation during embryonic development, wound healing and immune responses all require the orchestrated movement of cells in particular directions to specific locations...

. The three main pathways that lead to its activation are GPCR pathways, RTK pathways and gated ion channels; it regulates proteins either directly or by binding to an enzyme.

Lipophilics


Lipophilic second messenger molecules are derived from lipids residing in cellular membranes; enzymes stimulated by activated receptors activate the lipids by modifying them. Examples include diacylglycerol
Diglyceride
A diglyceride, or a diacylglycerol , is a glyceride consisting of two fatty acid chains covalently bonded to a glycerol molecule through ester linkages....

 and ceramide
Ceramide
Ceramides are a family of lipid molecules. A ceramide is composed of sphingosine and a fatty acid. Ceramides are found in high concentrations within the cell membrane of cells. They are one of the component lipids that make up sphingomyelin, one of the major lipids in the lipid bilayer...

, the former required for the activation of protein kinase C
Protein kinase C
Protein kinase C also known as PKC is a family of enzymes that are involved in controlling the function of other proteins through the phosphorylation of hydroxyl groups of serine and threonine amino acid residues on these proteins. PKC enzymes in turn are activated by signals such as increases in...

.

Nitric oxide


Nitric oxide (NO) acts as a second messenger because it is a free radical that can diffuse through the plasma membrane and affect nearby cells. It is synthesised from arginine
Arginine
Arginine is an α-amino acid. The L-form is one of the 20 most common natural amino acids. At the level of molecular genetics, in the structure of the messenger ribonucleic acid mRNA, CGU, CGC, CGA, CGG, AGA, and AGG, are the triplets of nucleotide bases or codons that codify for arginine during...

 and oxygen by the NO synthase and works through activation of soluble guanylyl cyclase
Soluble guanylyl cyclase
Soluble guanylyl cyclase is the only known receptor for nitric oxide, NO. It is soluble, i.e. completely intracellular. Most notably, this enzyme is involved in vasodilation...

, which when activated produces another second messenger, cGMP. NO can also act through covalent modification of proteins or their metal co-factors; some have a redox mechanism and are reversible. It is toxic in high concentrations and causes damage during stroke
Stroke
A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...

, but is the cause of many other functions like relaxation of blood vessels, apoptosis
Apoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...

 and erection
Erection
Penile erection is a physiological phenomenon where the penis becomes enlarged and firm. Penile erection is the result of a complex interaction of psychological, neural, vascular and endocrine factors, and is usually, though not exclusively, associated with sexual arousal...

s.

Redox Signaling


In addition to nitric oxide, other electronically-activated species are also signal-transducing agents in a process called redox signaling
Redox signaling
Redox signaling is when free radicals, reactive oxygen species , and other electronically activated species such as nitric oxide act as biological messengers. Arguably, hydrogen sulfide and carbon monoxide are also redox signaling molecules...

. Examples include superoxide
Superoxide
A superoxide, also known by the obsolete name hyperoxide, is a compound that possesses the superoxide anion with the chemical formula O2−. The systematic name of the anion is dioxide. It is important as the product of the one-electron reduction of dioxygen O2, which occurs widely in nature...

, hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...

, carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

, and hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

. Redox signaling also includes active modulation of electonic flows in semiconductive macromolecules.

Cellular responses


Gene activations and metabolism alterations are examples of cellular responses to extracellular stimulation that require signal transduction. Gene activation leads to further cellular effects, since the products of responding genes include instigators of activation; transcription factors produced as a result of a signal transduction cascade can activate even more genes. Hence, an initial stimulus can trigger the expression of a large number of genes, leading to physiological events like the increased uptake of glucose from the blood stream and the migration of neutrophils to sites of infection. The set of genes and their activation order to certain stimuli is referred to as a genetic program
Genetic program
In biology, a genetic program of a cell is a physiological change brought about by a temporal pattern of activation of a particular subset of genes....

.

Mammalian cells require stimulation for cell division and survival; in the absence of growth factor
Growth factor
A growth factor is a naturally occurring substance capable of stimulating cellular growth, proliferation and cellular differentiation. Usually it is a protein or a steroid hormone. Growth factors are important for regulating a variety of cellular processes....

, apoptosis
Apoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...

 ensues. Such requirements for extracellular stimulation are necessary for controlling cell behavior in unicellular and multicellular organisms; signal transduction pathways are perceived to be so central to biological processes that a large number of diseases are attributed to their disregulation.

Major pathways


Following are some major signaling pathways, demonstrating how ligands binding to their receptors can affect second messengers and eventually result in altered cellular responses.
  • MAPK/ERK pathway
    MAPK/ERK pathway
    The MAPK/ERK pathway is a chain of proteins in the cell that communicates a signal from a receptor on the surface of the cell to the DNA in the nucleus of the cell. The signal starts when a growth factor binds to the receptor on the cell surface and ends when the DNA in the nucleus expresses a...

    : A pathway that couples intracellular responses to the binding of growth factor
    Growth factor
    A growth factor is a naturally occurring substance capable of stimulating cellular growth, proliferation and cellular differentiation. Usually it is a protein or a steroid hormone. Growth factors are important for regulating a variety of cellular processes....

    s to cell
    Cell (biology)
    The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

     surface receptor
    Receptor (biochemistry)
    In biochemistry, a receptor is a molecule found on the surface of a cell, which receives specific chemical signals from neighbouring cells or the wider environment within an organism...

    s.  This pathway is very complex and includes many protein
    Protein
    Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

     components.  In many cell types, activation of this pathway promotes cell division
    Cell division
    Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...

    , and many forms of cancer
    Cancer
    Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...

     are associated with aberrations in it.

  • cAMP dependent pathway
    CAMP dependent pathway
    In the field of molecular biology, the cAMP-dependent pathway, also known as the adenylyl cyclase pathway, is a G protein-coupled receptor-triggered signaling cascade used in cell communication.-Mechanism:...

    : In humans, cAMP works by activating protein kinase A (PKA, cAMP-dependent protein kinase
    CAMP-dependent protein kinase
    In cell biology, Protein kinase A refers to a family of enzymes whose activity is dependent on cellular levels of cyclic AMP . PKA is also known as cAMP-dependent protein kinase...

    ) (see picture), and thus, further effects mainly depend on cAMP-dependent protein kinase, which vary based on the type of cell.

  • IP3/DAG pathway: PLC cleaves the phospholipid
    Phospholipid
    Phospholipids are a class of lipids that are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline; one exception to this rule is sphingomyelin, which is derived from...

     phosphatidylinositol 4,5-bisphosphate
    Phosphatidylinositol 4,5-bisphosphate
    Phosphatidylinositol 4,5-bisphosphate or PtdInsP2, also known simply as PIP2, is a minor phospholipid component of cell membranes...

     (PIP2) yielding diacyl glycerol (DAG) and inositol 1,4,5-triphosphate (IP3).  DAG remains bound to the membrane, and IP3 is released as a soluble structure into the cytosol
    Cytosol
    The cytosol or intracellular fluid is the liquid found inside cells, that is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into compartments....

    .  IP3 then diffuses through the cytosol to bind to IP3 receptors
    Inositol triphosphate receptor
    Inositol trisphosphate receptor is a membrane glycoprotein complex acting as Ca2+ channel activated by inositol trisphosphate . InsP3R is very diverse among organisms, and is necessary for the control of cellular and physiological processes including cell division, cell proliferation, apoptosis,...

    , particular calcium channel
    Calcium channel
    A Calcium channel is an ion channel which displays selective permeability to calcium ions. It is sometimes synonymous as voltage-dependent calcium channel, although there are also ligand-gated calcium channels.-Comparison tables:...

    s in the endoplasmic reticulum
    Endoplasmic reticulum
    The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...

     (ER).  These channels are specific to calcium
    Calcium
    Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

     and only allow the passage of calcium to move through.  This causes the cytosolic concentration of Calcium to increase, causing a cascade of intracellular changes and activity.  In addition, calcium and DAG together works to activate PKC, which goes on to phosphorylate other molecules, leading to altered cellular activity.  End effects include taste, manic depression, tumor promotion, etc.

See also

  • Two-component regulatory system
  • Functional selectivity
    Functional Selectivity
    Functional selectivity is the ligand-dependent selectivity for certain signal transduction pathways in one and the same receptor. This can be present when a receptor has several possible signal transduction pathways...

  • G protein-coupled receptor
    G protein-coupled receptor
    G protein-coupled receptors , also known as seven-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptor, and G protein-linked receptors , comprise a large protein family of transmembrane receptors that sense molecules outside the cell and activate inside signal...

  • GTPase
    GTPase
    GTPases are a large family of hydrolase enzymes that can bind and hydrolyze guanosine triphosphate . The GTP binding and hydrolysis takes place in the highly conserved G domain common to all GTPases.-Functions:...

    s
  • Protein phosphatase
  • Redox signaling
    Redox signaling
    Redox signaling is when free radicals, reactive oxygen species , and other electronically activated species such as nitric oxide act as biological messengers. Arguably, hydrogen sulfide and carbon monoxide are also redox signaling molecules...

  • Transduction
    Transduction (genetics)
    Transduction is the process by which DNA is transferred from one bacterium to another by a virus. It also refers to the process whereby foreign DNA is introduced into another cell via a viral vector. Transduction does not require cell-to-cell contact , and it is DNAase resistant...


External links