Contents 1 Discovery 2 Structure 3 Role in the cell cycle 3.1 Activation 3.2 Overview of functions 3.3 Targets 3.4 Inhibition of myosin 3.5 Disassembly by anaphase-promoting complex 4 References

Discovery[edit] In 1971, two independent teams of researchers (Yoshio Masui 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 with progesterone.[3][4] 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. Evidence that a diffusible factor regulates the entry into mitosis had been previously obtained in 1966 using the slime mold Physarum polycephalum in which the nuclei of the multi-nucleate plasmodial form undergo synchronous mitoses. Fusing plasmodia whose cell cycles were out of phase with each other led to a synchronous mitosis in the next mitotic cycle. This result demonstrated that mitotic entry was controlled by a diffusible cytoplasmic factor and not by a "nuclear clock." [5]

Structure[edit] MPF is composed of two subunits: Cyclin-dependent kinase 1 (CDK1), the cyclin-dependent kinase subunit. It uses ATP to phosphorylate specific serine and threonine residues of target proteins. Cyclin, a regulatory subunit. The cyclins are necessary for the kinase subunit to function with the appropriate substrate. The mitotic cyclins can be grouped as cyclins A & B. These cyclins have a nine residue sequence in the N-terminal region called the “destruction box”, which can be recognized by the ubiquitin ligase enzyme which destroys the cyclins when appropriate.

Role in the cell cycle[edit] During G1 and S phase, the CDK1 subunit of MPF is inactive due to an inhibitory enzyme, Wee1. Wee1 phosphorylates the Thr-14 residues in yeast and Tyr-15 residues in humans of CDK1, rendering MPF inactive. During the transition of G2 to M phase, cdk1 is de-phosphorylated by CDC25. The CDK1 subunit is now free and can bind to cyclin B, activate MPF, and make the cell enter mitosis. There is also a positive feedback loop that inactivates wee1. [clarification needed] Activation[edit] MPF must be activated in order for the cell to transition from G2 to M phase. There are three 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 cyclin B is attached to CDK1. In addition, two other residues on the CDK1 subunit must be activated by dephosphorylation. CDC25 removes a phosphate from residues Threonine-14 (Thr-14) and Tyrosine-15 (Tyr-15) and adds a hydroxyl group. Cyclin B/CDK1 activates CDC25 resulting in a positive feedback loop. Overview of functions[edit] Triggers the formation of mitotic spindle through microtubule instability. Promotes mitosis i.e. chromatin condensation through phosphorylation of condensins. 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 the nuclear lamina & breakdown of nuclear envelope into small vesicles. Causes phosphorylation of GM130, which leads to the fragmentation of the Golgi and the ER Targets[edit] The following are affected by MPF. condensins, which enable chromatin condensation (see prophase) various microtubule-associated proteins involved in mitotic spindle formation lamins, interaction contributing to degradation of the nuclear envelope Histones, H1 and H3 Golgi matrix, to cause fragmentation Inhibition of myosin[edit] MPF phosphorylates inhibitory sites on myosin early in mitosis. This prevents cytokinesis. When MPF activity falls at anaphase, the inhibitory sites are dephosphorylated and cytokinesis proceeds. Disassembly by anaphase-promoting complex[edit] MPF is disassembled when anaphase-promoting complex (APC) polyubiquitinates cyclin B, marking it for degradation in a negative feedback loop. 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. As the concentration of Cyclin B/CDK1 increases, the heterodimer promotes APC to polyubiquitinate Cyclin B/CDK1.

References[edit] ^ Smith, L.Dennis; Ecker, R.E. (May 1971). "The interaction of steroids with Rana pipiens oocytes in the induction of maturation". Developmental Biology. 25 (2): 232–247. doi:10.1016/0012-1606(71)90029-7.  ^ Masui, Yoshio; Markert, Clement L. (May 1971). "Cytoplasmic control of nuclear behavior during meiotic maturation of frog oocytes". Journal of Experimental Zoology. 177 (2): 129–145. doi:10.1002/jez.1401770202.  ^ Masui Y, Markert CL (1971), Cytoplasmic control of nuclear behavior during meiotic maturation of frog oocytes. J Exp Zool 177:2, 129-145 ^ Smith LD, Ecker RE (1971), The interaction of steroids with Rana pipiens oocytes in the induction of maturation. Dev Biol 25:2, 232-247 ^ Rusch HP, Sachsenmaier W, Behrens K, Gruter V. (1966). Synchronization of mitosis by the fusion of the plasmodia of Physarum polycephalum. J. Cell Biol.31:204-9. v t e Cell cycle proteins Cyclin A (A1, A2) B (B1, B2, B3) D (D1, D2, D3) E (E1, E2) CDK 1 2 3 4 5 6 7 8 9 10 CDK-activating kinase CDK inhibitor INK4a/ARF (p14arf/p16, p15, p18, p19) cip/kip (p21, p27, p57) P53 p63 p73 family p53 p63 p73 Other Cdc2 Cdc25 Cdc42 Cellular apoptosis susceptibility protein E2F Maturation promoting factor Wee Cullin (CUL7) Phases and checkpoints Interphase G1 phase S phase G2 phase M phase Mitosis (Preprophase Prophase Prometaphase Metaphase Anaphase Telophase) Cytokinesis Cell cycle checkpoints Restriction point Spindle checkpoint Postreplication checkpoint Other cellular phases Apoptosis G0 phase Meiosis Retrieved from "" Categories: Protein complexesCell cycleHidden categories: Articles needing additional references from September 2014All articles needing additional referencesWikipedia articles needing clarification from November 2012

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