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Transmembrane protein

Transmembrane protein

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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 in the case of waste byproducts. As a response to the shape of certain molecules these "freight handling" TPs may have special ways of folding up or bending that will move a substance through the biological membrane.

A transmembrane protein is a polytopic 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...

 that spans an entire biological membrane
Biological membrane
A biological membrane or biomembrane is an enclosing or separatingmembrane that acts as a selective barrier, within or around a cell. It consists of a lipid bilayer with embedded proteins that may constitute close to 50% of membrane content...

. Transmembrane proteins aggregate and precipitate in water. They require detergent
Detergent
A detergent is a surfactant or a mixture of surfactants with "cleaning properties in dilute solutions." In common usage, "detergent" refers to alkylbenzenesulfonates, a family of compounds that are similar to soap but are less affected by hard water...

s or nonpolar solvents for extraction, although some of them (beta-barrels) can be also extracted using denaturing agents.

All transmembrane proteins are integral membrane protein
Integral membrane protein
An integral membrane protein is a protein molecule that is permanently attached to the biological membrane. Proteins that cross the membrane are surrounded by "annular" lipids, which are defined as lipids that are in direct contact with a membrane protein...

s, but not all IMPs are transmembrane proteins.

Types


There are two basic types of transmembrane proteins:
  1. Alpha-helical. These proteins are present in the inner membranes of bacterial cells or the plasma membrane of eukaryotes, and sometimes in the outer membrane
    Outer membrane
    The bacterial outer membrane is found in Gram-negative bacteria. Its composition is distinct from that of the cytoplasmic membrane - among other things, the outer leaflet of the membrane includes a complex lipopolysaccharide whose lipid portion acts as an endotoxin - and it is linked to the cell's...

    s. This is the major category of transmembrane proteins. In humans, 27% of all proteins have been estimated to be alpha-helical membrane proteins.
  2. Beta-barrels. These proteins are so far found only in outer membrane
    Outer membrane
    The bacterial outer membrane is found in Gram-negative bacteria. Its composition is distinct from that of the cytoplasmic membrane - among other things, the outer leaflet of the membrane includes a complex lipopolysaccharide whose lipid portion acts as an endotoxin - and it is linked to the cell's...

    s of Gram-negative bacteria, cell wall
    Cell wall
    The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...

     of Gram-positive bacteria, and outer membrane
    Outer membrane
    The bacterial outer membrane is found in Gram-negative bacteria. Its composition is distinct from that of the cytoplasmic membrane - among other things, the outer leaflet of the membrane includes a complex lipopolysaccharide whose lipid portion acts as an endotoxin - and it is linked to the cell's...

    s of mitochondria and chloroplasts. All beta-barrel transmembrane proteins have simplest up-and-down topology, which may reflect their common evolutionary origin and similar folding mechanism.

Another classification refers to the position of the N- and C-terminal domains. Types I, II, and III are single pass molecules, while type IV are multiple pass molecules. Type I transmembrane proteins are anchored to the lipid membrane with a stop-transfer anchor sequence and have their N-terminal domains targeted to the ER lumen during synthesis (and the extracellular space, if mature forms are located on plasmalemma). Type II and III are anchored with a signal-anchor sequence, with type II being targeted to the ER lumen with its C-terminal domain, while type III have their N-terminal domains targeted to the ER lumen. Type IV is subdivided into IV-A, with their N-terminal domains targeted to the cytosol and IV-B, with an N-terminal domain targeted to the lumen. The implications for the division in the four types are especially manifest at the time of translocation and ER-bound translation, when the protein has to be passed through the ER membrane in a direction dependent on the type.

Stability of α-helical transmembrane proteins


Transmembrane α-helical
Transmembrane helix
Transmembrane domain usually denotes a single transmembrane alpha helix of a transmembrane protein. It is called a "domain" because an alpha-helix in a membrane can fold independently from the rest of the protein, similar to domains of water-soluble proteins...

 proteins are unusually stable judging from thermal denaturation
Denaturation (biochemistry)
Denaturation is a process in which proteins or nucleic acids lose their tertiary structure and secondary structure by application of some external stress or compound, such as a strong acid or base, a concentrated inorganic salt, an organic solvent , or heat...

 studies, because they do not unfold completely within the membranes (the complete unfolding would require breaking down too many α-helical H-bonds in the nonpolar media). On the other hand, these proteins easily misfold, due to non-native aggregation in membranes, transition to the molten globule
Molten globule
The term molten globule was first coined by A. Wada and M Ohgushi in 1983. It was first found in cytochrome c, which conserves a native-like secondary structure content but without the tightly packed protein interior, under low pH and high salt concentration...

 states, formation of non-native disulfide bonds, or unfolding of peripheral regions and nonregular loops that are locally less stable.

It is also important to properly define the unfolded state. The unfolded state of membrane proteins in detergent
Detergent
A detergent is a surfactant or a mixture of surfactants with "cleaning properties in dilute solutions." In common usage, "detergent" refers to alkylbenzenesulfonates, a family of compounds that are similar to soap but are less affected by hard water...

 micelles is different from that in the thermal denaturation
Denaturation (biochemistry)
Denaturation is a process in which proteins or nucleic acids lose their tertiary structure and secondary structure by application of some external stress or compound, such as a strong acid or base, a concentrated inorganic salt, an organic solvent , or heat...

 experiments. This state represents a combination of folded hydrophobic α-helices and partially unfolded segments covered by the detergent. For example, the "unfolded" bacteriorhodopsin
Bacteriorhodopsin
Bacteriorhodopsin is a protein used by Archaea, the most notable one being Halobacteria. It acts as a proton pump; that is, it captures light energy and uses it to move protons across the membrane out of the cell...

 in SDS
Sodium dodecyl sulfate
Sodium dodecyl sulfate , sodium laurilsulfate or sodium lauryl sulfate is an organic compound with the formula CH311OSO3Na). It is an anionic surfactant used in many cleaning and hygiene products...

 micelles has four transmembrane α-helices folded, while the rest of the protein is situated at the micelle-water interface and can adopt different types of non-native amphiphilic structures. Free energy differences between such detergent-denatured and native states are similar to stabilities of water-soluble proteins (< 10 kcal/mol).

Folding of α-helical transmembrane proteins


Refolding of α-helical transmembrane proteins in vitro is technically difficult. There are relatively few examples of the successful refolding experiments, as for bacteriorhodopsin
Bacteriorhodopsin
Bacteriorhodopsin is a protein used by Archaea, the most notable one being Halobacteria. It acts as a proton pump; that is, it captures light energy and uses it to move protons across the membrane out of the cell...

. In vivo all such proteins are normally folded co-translationally within the large transmembrane translocon
Translocon
The translocon is the complex of proteins associated with the translocation of nascent polypeptides across membranes. In eukaryotes the polypeptides are transported into the interior space of the endoplasmic reticulum from the cytosol...

.The translocon channel provides a highly heterogeneous environment for the nascent transmembane α-helices. A relatively polar amphiphilic α-helix can adopt a transmembrane orientation in the translocon (although it would be at the membrane surface or unfolded in vitro), because its polar residues can face the central water-filled channel of the translocon. Such mechanism is necessary for incorporation of polar α-helices into structures of transmembrane proteins. The amphiphilic helices remain attached to the translocon until the protein is completely synthesized and folded. If the protein remains unfolded and attached to the translocon for too long, it is degraded by specific "quality control" cellular systems.

Stability and folding of β-barrel transmembrane proteins


Stability of β-barrel transmembrane proteins is similar to stability of water-soluble proteins, based on chemical denaturation studies. Their folding in vivo is facilitated by water-soluble chaperones, such as protein Skp http://faculty.virginia.edu/tamm/pages/BBA_Folding_Review.pdf.

Light absorption-driven transporters

  • Bacteriorhodopsin
    Bacteriorhodopsin
    Bacteriorhodopsin is a protein used by Archaea, the most notable one being Halobacteria. It acts as a proton pump; that is, it captures light energy and uses it to move protons across the membrane out of the cell...

    -like proteins including rhodopsin
    Rhodopsin
    Rhodopsin, also known as visual purple, is a biological pigment of the retina that is responsible for both the formation of the photoreceptor cells and the first events in the perception of light. Rhodopsins belong to the G-protein coupled receptor family and are extremely sensitive to light,...

     (see also opsin
    Opsin
    Opsins are a group of light-sensitive 35–55 kDa membrane-bound G protein-coupled receptors of the retinylidene protein family found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical...

    )http://opm.phar.umich.edu/families.php?superfamily=6
  • Bacterial photosynthetic reaction centre
    Photosynthetic reaction centre
    A photosynthetic reaction center is a complex of several proteins, pigments and other co-factors assembled together to execute the primary energy conversion reactions of photosynthesis...

    s and photosystem
    Photosystem
    Photosystems are functional and structural units of protein complexes involved in photosynthesis that together carry out the primary photochemistry of photosynthesis: the absorption of light and the transfer of energy and electrons...

    s I and II http://opm.phar.umich.edu/families.php?superfamily=2
  • Light harvesting complexes from bacteria
    Bacteria
    Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

     and chloroplasts http://opm.phar.umich.edu/families.php?superfamily=1

Oxidoreduction-driven transporters

  • Transmembrane cytochrome b-like proteins http://opm.phar.umich.edu/families.php?superfamily=3: coenzyme Q - cytochrome c reductase
    Coenzyme Q - cytochrome c reductase
    In enzymology, a ubiquinol—cytochrome-c reductase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are dihydroquinone and ferri- cytochrome c, whereas its 3 products are quinone , ferro- cytochrome c, and H+.This enzyme belongs to the family of...

     (cytochrome bc1 ); cytochrome b6f complex
    Cytochrome b6f complex
    The cytochrome b6f complex is an enzyme found in the thylakoid membrane in chloroplasts of plants, cyanobacteria, and green algae, catalyzing the transfer of electrons from plastoquinol to plastocyanin...

    ; formate dehydrogenase, respiratory nitrate reductase
    Nitrate reductase
    Nitrate reductases are molybdoenzymes that reduce nitrate to nitrite .* Eukaryotic nitrate reductases are part of the sulfite oxidase family of molybdoenzymes....

    ; succinate - coenzyme Q reductase
    Succinate - coenzyme Q reductase
    Succinate dehydrogenase or succinate-coenzyme Q reductase or Complex II is an enzyme complex, bound to the inner mitochondrial membrane of mammalian mitochondria and many bacterial cells...

     (fumarate reductase); and succinate dehydrogenase. See electron transport chain
    Electron transport chain
    An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...

    .
  • Cytochrome c oxidase
    Cytochrome c oxidase
    The enzyme cytochrome c oxidase or Complex IV is a large transmembrane protein complex found in bacteria and the mitochondrion.It is the last enzyme in the respiratory electron transport chain of mitochondria located in the mitochondrial membrane...

    s http://opm.phar.umich.edu/families.php?superfamily=4 from bacteria
    Bacteria
    Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

     and mitochondria

Electrochemical potential-driven transporters

  • Proton or sodium translocating F-type and V-type ATPase
    ATPase
    ATPases are a class of enzymes that catalyze the decomposition of adenosine triphosphate into adenosine diphosphate and a free phosphate ion. This dephosphorylation reaction releases energy, which the enzyme harnesses to drive other chemical reactions that would not otherwise occur...

    s http://opm.phar.umich.edu/families.php?superfamily=5

P-P-bond hydrolysis-driven transporters

  • P-type calcium ATPase
    Calcium ATPase
    Calcium ATPase is a form of P-ATPase that transfers calcium after a muscle has contracted. The calcium ATPase are:*Plasma membrane Ca2+ ATPase *Sarcoplasmic reticulum Ca2+ ATPase - Plasma membrane Ca2+ ATPase :...

      (five different conformations) http://opm.phar.umich.edu/families.php?superfamily=22
  • Calcium ATPase regulators phospholamban
    Phospholamban
    Phospholamban, also known as PLN, is a protein that in humans is encoded by the PLN gene. Phospholamban is a 52-amino acid integral membrane protein that regulates the Ca2+ pump in cardiac muscle and skeletal muscle cells.- Function :...

     and sarcolipin http://opm.phar.umich.edu/families.php?superfamily=70
  • ABC transporters: BtuCD, multidrug transporter, and molybdate uptake transporter
  • General secretory pathway
    Secretory pathway
    The secretory pathway is a series of steps a cell uses to move proteins out of the cell; a process known as secretion. The path of a protein destined for secretion has its origins in the rough endoplasmic reticulum, a membrane-bound compartment in the cell...

     (Sec) translocon
    Translocon
    The translocon is the complex of proteins associated with the translocation of nascent polypeptides across membranes. In eukaryotes the polypeptides are transported into the interior space of the endoplasmic reticulum from the cytosol...

     (preprotein translocase SecY) http://opm.phar.umich.edu/families.php?superfamily=19

Porters (uniporters, symporters, antiporters)


Alpha-helical channels including ion channels

  • Voltage-gated ion channel
    Voltage-gated ion channel
    Voltage-gated ion channels are a class of transmembrane ion channels that are activated by changes in electrical potential difference near the channel; these types of ion channels are especially critical in neurons, but are common in many types of cells....

     like, including potassium channel
    Potassium channel
    In the field of cell biology, potassium channels are the most widely distributed type of ion channel and are found in virtually all living organisms. They form potassium-selective pores that span cell membranes...

    s KcsA and KvAP, and inward-rectifier potassium ion channel
    Inward-rectifier potassium ion channel
    Inwardly rectifying potassium channels are a specific subset of potassium selective ion channels. To date, seven subfamilies have been identified in various mammalian cell types...

     Kirbac http://opm.phar.umich.edu/families.php?superfamily=8
  • Large-conductance mechanosensitive channel, MscL http://opm.phar.umich.edu/families.php?superfamily=12
  • Small-conductance mechanosensitive ion channel (MscS)
    Mechanosensitive ion channel
    Mechanosensitive channels are found in a number of tissues and organisms and are thought to be the sensors for a number of systems including the senses of touch, hearing and balance, as well as participating in cardiovascular regulation and osmotic homeostasis...

     http://opm.phar.umich.edu/families.php?superfamily=11
  • CorA metal ion transporters  http://opm.phar.umich.edu/families.php?superfamily=72
  • Ligand-gated ion channel
    Ligand-gated ion channel
    Ligand-gated ion channels are one type of ionotropic receptor or channel-linked receptor. They are a group of transmembrane ion channels that are opened or closed in response to the binding of a chemical messenger , such as a neurotransmitter.The binding site of endogenous ligands on LGICs...

     of neurotransmitter
    Neurotransmitter
    Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...

     receptors (acetylcholine receptor
    Acetylcholine receptor
    An acetylcholine receptor is an integral membrane protein that responds to the binding of acetylcholine, a neurotransmitter.-Classification:...

    ) http://opm.phar.umich.edu/families.php?superfamily=14
  • Aquaporin
    Aquaporin
    Aquaporins are proteins embedded in the cell membrane that regulate the flow of water.Aquaporins are integral membrane proteins from a larger family of major intrinsic proteins that form pores in the membrane of biological cells....

    s http://opm.phar.umich.edu/families.php?superfamily=7
  • Chloride channel
    Chloride channel
    Chloride channels are a superfamily of poorly understood ion channels consisting of approximately 13 members.Chloride channels display a variety of important physiological and cellular roles that include regulation of pH, volume homeostasis, organic solute transport, cell migration, cell...

    s http://opm.phar.umich.edu/families.php?superfamily=10
  • Outer membrane auxiliary proteins (polysaccharide transporter) http://opm.phar.umich.edu/families.php?superfamily=188 - α-helical transmembrane proteins from the outer bacterial membrane

Enzymes


Proteins with alpha-helical transmembrane anchors


β-barrels composed of a single polypeptide chain

  • Beta barrels from eight beta-strands and with "shear number" of ten (n=8, S=10) http://opm.phar.umich.edu/families.php?superfamily=26. They include:
    • OmpA-like transmembrane domain
      OmpA-like transmembrane domain
      OmpA-like transmembrane domain is an evolutionarily conserved domain of outer membrane proteins. This domain consists of an eight-stranded beta barrel. OmpA is the predominant cell surface antigen in enterobacteria found in about 100,000 copies per cell. The expression of OmpA is tightly regulated...

       (OmpA),
    • Virulence-related outer membrane protein family
      Virulence-related outer membrane protein family
      Virulence-related outer membrane proteins are expressed in Gram-negative bacteria and are essential to bacterial survival within macrophages and for eukaryotic cell invasion.This family consists of several bacterial and phage Ail/Lom-like proteins....

       (OmpX),
    • Outer membrane protein W family
      Outer membrane protein W family
      Outer membrane protein W family is a family of evolutionarily related proteins from the outer bacterial membrane.This family includes outer membrane protein W proteins from a variety of bacterial species. This protein may form the receptor for S4 colicins in Escherichia coli....

       (OmpW),
    • Antimicrobial peptide resistance and lipid A acylation protein family
      Antimicrobial peptide resistance and lipid A acylation protein family
      Antimicrobial peptide resistance and lipid A acylation protein PagP is a family of several bacterial antimicrobial peptide resistance and lipid A acylation proteins. The bacterial outer membrane enzyme PagP transfers a palmitate chain from a phospholipid to lipid A...

       (PagP)
    • Lipid A deacylase PagL
      Lipid A deacylase PagL
      Lipid A deacylase is an outer membrane protein with lipid A 3-O-deacylase activity. It forms an 8 stranded beta barrel structure.-References:* Crystal structure and catalytic mechanism of the LPS 3-O-deacylase PagL from Pseudomonas aeruginosa...

      , and
    • Opacity family porins
      Opacity family porins
      Opacity family porins are a family of porins from pathogenic Neisseria.These bacteria possess a repertoire of phase-variable opacity proteins that mediate various pathogen/host cell interactions. These proteins are related to OmpA-like transmembrane domain family....

       (NspA)
  • Autotransporter domain
    Autotransporter domain
    In molecular biology an autotransporter domain is a structural domain found in some outer membrane proteins.Translocation of polypeptide chains through the outer membrane of Gram-negative bacteria is termed secretion. Secretion occurs via a number of different pathways in this type of bacterium...

     (n=12,S=14') http://opm.phar.umich.edu/families.php?superfamily=28
  • FadL outer membrane protein transport family
    FadL outer membrane protein transport family
    Outer membrane transport proteins family includes several proteins that are involved in toluene catabolism and degradation of aromatic hydrocarbons. This family also includes protein FadL involved in translocation of long-chain...

    , including Fatty acid
    Fatty acid
    In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...

     transporter FadL (n=14,S=14) http://opm.phar.umich.edu/families.php?superfamily=30
  • General bacterial porin family
    General bacterial porin family
    General bacterial porins are a family of proteins from the outer membrane of Gram-negative bacteria. The porins act as molecular filters for hydrophilic compounds. They are responsible for the 'molecular sieve' properties of the outer membrane. Porins form large water-filled channels which allow...

    , known as trimeric porins
    Porin (protein)
    Porins are beta barrel proteins that cross a cellular membrane and act as a pore through which molecules can diffuse. Unlike other membrane transport proteins, porins are large enough to allow passive diffusion, i.e., they act as channels that are specific to different types of molecules...

     (n=16,S=20) http://opm.phar.umich.edu/families.php?superfamily=31
  • Maltoporin
    Maltoporin
    Maltoporins are a family of outer membrane proteins. Maltoporin forms a trimeric structure which facilitates the diffusion of maltodextrins across the outer membrane of Gram-negative bacteria. The membrane channel is formed by an antiparallel beta-barrel....

    , or sugar porins
    Porin (protein)
    Porins are beta barrel proteins that cross a cellular membrane and act as a pore through which molecules can diffuse. Unlike other membrane transport proteins, porins are large enough to allow passive diffusion, i.e., they act as channels that are specific to different types of molecules...

     (n=18,S=22) http://opm.phar.umich.edu/families.php?superfamily=32
  • Nucleoside-specific porin
    Nucleoside-specific porin
    Nucleoside-specific porin is an outer membrane protein, Tsx, which constitutes the receptor for colicin K and Bacteriophage T6, and functions as a substrate-specific channel for nucleosides and deoxy-nucleosides. The protein contains 294 amino acids, the first 22 of which are characteristic of a...

     (n=12,S=16) http://opm.phar.umich.edu/families.php?superfamily=71
  • Outer membrane phospholipase A1
    Outer membrane phospholipase A1
    Outer membrane phospholipase A1 is an acyl hydrolasewith a broad substrate specificity from the bacterial outer membrane. It has been proposed that Ser164 is the active site of the protein...

    (n=12,S=16) http://opm.phar.umich.edu/families.php?superfamily=29
  • TonB-dependent receptors
    TonB-dependent receptors
    TonB-dependent receptors is a family of beta-barrel proteins from the outer membrane of Gram-negative bacteria. The TonB complex senses signals from outside the bacterial cell and transmits them via two membranes into the cytoplasm, leading to transcriptional activation of target genes.In...

     and their plug domain. They are ligand-gated outer membrane channels (n=22,S=24), including cobalamin transporter BtuB, Fe(III)-pyochelin receptor FptA, receptor FepA, ferric hydroxamate uptake receptor FhuA, transporter FecA, and pyoverdine receptor FpvA http://opm.phar.umich.edu/families.php?superfamily=33
  • Outer membrane protein OpcA
    Outer membrane protein OpcA
    Outer membrane adhesin OpcA protein family consists of several Neisseria species specific outer membrane proteins. Neisseria meningitidis causes meningococcal meningitis and septicemia...

     family (n=10,S=12) that includes outer membrane protease
    Protease
    A protease is any enzyme that conducts proteolysis, that is, begins protein catabolism by hydrolysis of the peptide bonds that link amino acids together in the polypeptide chain forming the protein....

     OmpT and adhesin
    Adhesin
    Adhesins are cell-surface components or appendages of bacteria that facilitate bacterial adhesion or adherence to other cells or to inanimate surfaces. Adhesins are a type of virulence factor....

    /invasin OpcA protein http://opm.phar.umich.edu/families.php?superfamily=27
  • Outer membrane protein G
    Outer membrane protein G
    Outer membrane protein G is a porin, a channel proteins in the outer membrane of Gram-negative bacteria.Escherichia coli OmpG forms a 14-stranded beta-barrel and in contrast to most porins, appears to function as a monomer. The central pore of OmpG is wider than other E...

     porin family (n=14,S=16) http://opm.phar.umich.edu/families.php?superfamily=181


Note: n and S are, respectively, the number of beta-strands and the "shear number" of the beta-barrel

β-barrels composed of several polypeptide chains



See also Gramicidin
Gramicidin
Gramicidin is a heterogeneous mixture of six antibiotic compounds, gramicidins A, B and C, making up 80%, 6%,and 14% respectively, all of which are obtained from the soil bacterial species Bacillus brevis and called collectively gramicidin D. Gramicidin D are linear pentadecapeptides; that is...

 A http://opm.phar.umich.edu/protein.php?pdbid=1grm, a peptide that forms a dimeric transmembrane β-helix. It is also secreted by Gram-positive
Gram-positive
Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain and appearing red or pink...

 bacteria.

See also

  • cell membrane
    Cell membrane
    The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...

  • transmembrane receptors
  • membrane topology
    Membrane topology
    In biochemistry, the membrane topology of an transmembrane protein describes which portions of the amino-acid sequence of the protein lie within the plane of the surrounding lipid bilayer and which portions protrude into the watery environment on either side...

  • transmembrane helix
    Transmembrane helix
    Transmembrane domain usually denotes a single transmembrane alpha helix of a transmembrane protein. It is called a "domain" because an alpha-helix in a membrane can fold independently from the rest of the protein, similar to domains of water-soluble proteins...

  • membrane protein
    Membrane protein
    A membrane protein is a protein molecule that is attached to, or associated with the membrane of a cell or an organelle. More than half of all proteins interact with membranes.-Function:...

  • integral membrane protein
    Integral membrane protein
    An integral membrane protein is a protein molecule that is permanently attached to the biological membrane. Proteins that cross the membrane are surrounded by "annular" lipids, which are defined as lipids that are in direct contact with a membrane protein...

  • peripheral membrane protein
    Peripheral membrane protein
    Peripheral membrane proteins are proteins that adhere only temporarily to the biological membrane with which they are associated. These molecules attach to integral membrane proteins, or penetrate the peripheral regions of the lipid bilayer. The regulatory protein subunits of many ion channels and...

  • intramembrane protease
    Intramembrane protease
    Intramembrane proteases are enzymes that have the property of cleaving transmembrane domains of integral membrane proteins. All known intramembrane proteases are themselves integral membrane proteins with multiple transmembrane domains, and they have their active sites buried within the lipid...


Transporter Classification database