Maturation promoting factor
Encyclopedia
Maturation-promoting factor (abbreviated MPF, also called mitosis-promoting factor or M-Phase-promoting factor) is a heterodimeric
protein
composed of cyclin B
and cyclin-dependent kinase
(CDK1
, also known as Cdc2 or p34 kinase) that stimulates the mitotic
and meiotic
cell cycle
s. MPF promotes the entrance into mitosis from the G2 phase
by phosphorylating multiple proteins needed during mitosis. MPF is activated at the end of G2 by a phosphatase
, which removes an inhibitory phosphate group added earlier.
MPF is made of 2 subunits:
Targets of MPF include:
early in mitosis. This prevents cytokinesis
. When MPF activity falls at anaphase, the inhibitory sites are dephosphorylated and cytokinesis proceeds.
In addition, two other residues on the cdk1 subunit must be activated. cdc25 removes a phosphate on residues Threonine-14 (Thr-14) and Tyrosine-15 (Tyr-15) and adds a hydroxy group. CyclinB/cdk1 activates cdc25 resulting in a positive feedback loop.
During G1 and S phase, the MPF subunit cdk1 is inactive due to an inhibitory enzyme, Wee1. Wee1 phosphorylizes the Thr-14 and Tyr-15 residues. During the transition of G2 to M phase, MPF inhibits Wee1, again resulting in a positive feedback loop.
(APC) polyubiquitinates cyclin B, marking it for degradation in a negative feedback loop. As the concentration of CyclinB/cdk1 increases, the heterodimer promotes APC to polyubiquitinate CyclinB/cdk1.
and Clement Markert
, as well as Dennis Smith and Robert Ecker) found that frog oocytes arrested in G2 could be induced to enter M phase by microinjection of cytoplasm from oocytes that had been hormonally stimulated.
Because the entry of oocytes into meiosis is frequently referred to as oocyte maturation, this cytoplasmic factor was called maturation promoting factor (MPF). Further studies showed, however, that the activity of MPF is not restricted to the entry of oocytes into meiosis. To the contrary, MPF is also present in somatic cells, where it induces entry into M phase of the mitotic cycle .
The MPF is also called the M phase kinase because of its ability to phosphorylate target proteins at a specific point in the cell cycle and thus controlling their ability to function.
The MPF has two subunits: The Cyclins and the Cyclin dependent Kinases(CDK)
The cyclins are the regulatory subunits that are necessary for the kinase to function with appropriate substrate. The mitotic cyclins can be grouped as cyclins A & B.
The CDK is the catalaytic subunit that phosphorylates serine and threonine residues in target proteins.
In intact cells, cyclin degradation begins shortly after the onset of anaphase (late anaphase), the period of mitosis when sister chromatids are separated and pulled toward opposite spindle poles.
Biochemical studies with Xenopus egg extracts showed that after their synthesis, wild-type mitotic cyclins are modified by addition of ubiquitin, a highly conserved, 76-residue protein. Covalent attachment of chains of ubiquitin, a process called polyubiquitination, marks proteins for rapid degradation in eukaryotic cells by proteasomes, multiprotein cylindrical structures containing numerous proteases
Protein dimer
In biochemistry, a dimer is a macromolecular complex formed by two, usually non-covalently bound, macromolecules like proteins or nucleic acids...
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...
composed of cyclin B
Cyclin
Cyclins are a family of proteins that control the progression of cells through the cell cycle by activating cyclin-dependent kinase enzymes.- Function :...
and cyclin-dependent kinase
Cyclin-dependent kinase
thumb|350px|Schematic of the cell cycle. outer ring: I=[[Interphase]], M=[[Mitosis]]; inner ring: M=Mitosis; G1=[[G1 phase|Gap phase 1]]; S=[[S phase|Synthesis]]; G2=[[G2 phase|Gap phase 2]]...
(CDK1
Cyclin-dependent kinase
thumb|350px|Schematic of the cell cycle. outer ring: I=[[Interphase]], M=[[Mitosis]]; inner ring: M=Mitosis; G1=[[G1 phase|Gap phase 1]]; S=[[S phase|Synthesis]]; G2=[[G2 phase|Gap phase 2]]...
, also known as Cdc2 or p34 kinase) that stimulates the mitotic
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
and meiotic
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
cell cycle
Cell cycle
The cell cycle, or cell-division cycle, is the series of events that takes place in a cell leading to its division and duplication . In cells without a nucleus , the cell cycle occurs via a process termed binary fission...
s. MPF promotes the entrance into mitosis from the G2 phase
G2 phase
G2 phase is the 3rd and final subphase of Interphase in the cell cycle directly preceding Mitosis. It follows the successful completion of S phase, during which the cell’s DNA is replicated...
by phosphorylating multiple proteins needed during mitosis. MPF is activated at the end of G2 by a phosphatase
Phosphatase
A phosphatase is an enzyme that removes a phosphate group from its substrate by hydrolysing phosphoric acid monoesters into a phosphate ion and a molecule with a free hydroxyl group . This action is directly opposite to that of phosphorylases and kinases, which attach phosphate groups to their...
, which removes an inhibitory phosphate group added earlier.
MPF is made of 2 subunits:
- a subunit that transfers phosphate groups from ATPAdenosine triphosphateAdenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
to specific serineSerineSerine 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:...
and threonineThreonineThreonine is an α-amino acid with the chemical formula HO2CCHCHCH3. Its codons are ACU, ACA, ACC, and ACG. This essential amino acid is classified as polar...
residues of specific proteins (kinaseKinaseIn 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...
activity) - cyclinCyclinCyclins are a family of proteins that control the progression of cells through the cell cycle by activating cyclin-dependent kinase enzymes.- Function :...
, a regulatory subunit
Targets of MPF include:
- condensinCondensinCondensins are large protein complexes that play a central role in chromosome assembly and segregation in eukaryotic cells.- Subunit composition :...
s, which enable chromatinChromatinChromatin is the combination of DNA and proteins that make up the contents of the nucleus of a cell. The primary functions of chromatin are; to package DNA into a smaller volume to fit in the cell, to strengthen the DNA to allow mitosis and meiosis and prevent DNA damage, and to control gene...
condensation (see prophaseProphaseProphase, from the ancient Greek πρό and φάσις , is a stage of mitosis in which the chromatin condenses into a highly ordered structure called a chromosome in which the chromatin becomes visible. This process, called chromatin condensation, is mediated by the condensin complex...
) - various microtubuleMicrotubuleMicrotubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...
-associated proteins involved in mitotic spindleMitotic spindleIn cell biology, the spindle fibers are the structure that separates the chromosomes into the daughter cells during cell division. It is part of the cytoskeleton in eukaryotic cells...
formation - laminLaminNuclear lamins, also known as Class V intermediate filaments, are fibrous proteins providing structural function and transcriptional regulation in the cell nucleus. Nuclear lamins interact with membrane-associated proteins to form the nuclear lamina on the interior of the nuclear envelope...
s, interaction contributing to degradation of the nuclear envelopeNuclear envelopeA nuclear envelope is a double lipid bilayer that encloses the genetic material in eukaryotic cells. The nuclear envelope also serves as the physical barrier, separating the contents of the nucleus from the cytosol... - Histones, H1 and H3
- Golgi matrix, to cause fragmentation
Inhibition of myosin
MPF phosphorylates inhibitory sites on myosinMyosin
Myosins comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other eukaryotic motility processes. They are responsible for actin-based motility. The term was originally used to describe a group of similar...
early in mitosis. This prevents cytokinesis
Cytokinesis
Cytokinesis is the process in which the cytoplasm of a single eukaryotic cell is divided to form two daughter cells. It usually initiates during the late stages of mitosis, and sometimes meiosis, splitting a binucleate cell in two, to ensure that chromosome number is maintained from one generation...
. When MPF activity falls at anaphase, the inhibitory sites are dephosphorylated and cytokinesis proceeds.
Activation of MPF
MPF must be activated in order for the cell to transition from G2 to M phase. There are 3 amino acid residues responsible for this G2 to M phase transition. The threonine 161 (Thr-161) on cdk1 must be phosphorylated by a Cyclin Activating Kinase (CAK). CAK only phosphorylates Thr-161 when cyclinB is attached to cdk1.In addition, two other residues on the cdk1 subunit must be activated. cdc25 removes a phosphate on residues Threonine-14 (Thr-14) and Tyrosine-15 (Tyr-15) and adds a hydroxy group. CyclinB/cdk1 activates cdc25 resulting in a positive feedback loop.
During G1 and S phase, the MPF subunit cdk1 is inactive due to an inhibitory enzyme, Wee1. Wee1 phosphorylizes the Thr-14 and Tyr-15 residues. During the transition of G2 to M phase, MPF inhibits Wee1, again resulting in a positive feedback loop.
Disassembly by APC
MPF is disassembled when anaphase-promoting complexAnaphase-promoting complex
Anaphase-Promoting Complex, also called cyclosome , is an E3 ubiquitin ligase that marks target cell cycle proteins for degradation by the 26S proteasome. The APC/C is a large complex of 11–13 subunit proteins, including a cullin and RING subunit much like SCF...
(APC) polyubiquitinates cyclin B, marking it for degradation in a negative feedback loop. As the concentration of CyclinB/cdk1 increases, the heterodimer promotes APC to polyubiquitinate CyclinB/cdk1.
Regulator of Cell Cycle
In 1971, two independent teams of researchers (Yoshio MasuiYoshio Masui
is a Japanese cell biologist.Masui studied biology at Kyoto University, graduating with his B.Sc. in zoology in 1953, his M.Sc. in 1955 and his Ph.D. in 1961. While still studying at Kyoto University, he taught biology, first as a teacher's assistant and then as a teacher, at Konan University,...
and Clement Markert
Clement Markert
Clement Lawrence Markert was an American biologist credited with the discovery of isozymes...
, as well as Dennis Smith and Robert Ecker) found that frog oocytes arrested in G2 could be induced to enter M phase by microinjection of cytoplasm from oocytes that had been hormonally stimulated.
Because the entry of oocytes into meiosis is frequently referred to as oocyte maturation, this cytoplasmic factor was called maturation promoting factor (MPF). Further studies showed, however, that the activity of MPF is not restricted to the entry of oocytes into meiosis. To the contrary, MPF is also present in somatic cells, where it induces entry into M phase of the mitotic cycle .
The MPF is also called the M phase kinase because of its ability to phosphorylate target proteins at a specific point in the cell cycle and thus controlling their ability to function.
The MPF has two subunits: The Cyclins and the Cyclin dependent Kinases(CDK)
The cyclins are the regulatory subunits that are necessary for the kinase to function with appropriate substrate. The mitotic cyclins can be grouped as cyclins A & B.
The CDK is the catalaytic subunit that phosphorylates serine and threonine residues in target proteins.
Role of MPF
The MPF is a heterodimeric protein that act as the B type cyclins. These cyclins have a nine residue sequence in the N- terminal region called as the “destruction box”, which can be recognized by the ubiquitin ligase enzyme and thus destroys the cyclins.In intact cells, cyclin degradation begins shortly after the onset of anaphase (late anaphase), the period of mitosis when sister chromatids are separated and pulled toward opposite spindle poles.
Biochemical studies with Xenopus egg extracts showed that after their synthesis, wild-type mitotic cyclins are modified by addition of ubiquitin, a highly conserved, 76-residue protein. Covalent attachment of chains of ubiquitin, a process called polyubiquitination, marks proteins for rapid degradation in eukaryotic cells by proteasomes, multiprotein cylindrical structures containing numerous proteases
Main Function of MPF
- Triggers the formation of mitotic spindle.
- Promotes mitosis i.e. chromatin condensation.
- Causes nuclear envelope breakdown by phosphorylating the lamins that form an intermediate filament-type network (nuclear lamina) underlying the inner nuclear membrane.
- The three lamins present in the nuclear lamina, lamin A,B & C, are phosphorylated by MPF at serine amino residues. This leads to depolymerisation of nuclear lamina & breakdown of nuclear envelope into small vesicles.