P34 and cyclins

A 34 kDa protein (p34) plays an important function in the control of the cell cycle in all eukaryotes. It was first identified as the product of the cdc 2/cdc 28 gene in yeast mutants which caused cells to be arrested at a commitment point in G1 (Murray, 1981). However, anti-p34 antibodies injected into cells do not affect DNA synthesis but block cells in mitosis (Riabowol et al., 1989) and p34 is believed to function both at the onset of S-phase and at mitosis. 

The tyrosine dephosphorylation of p34 and phosphorylation of cyclin M precipitates cells into mitosis, while the formation of active MPF leads to phosphorylation of histone HI and the completion of, and exit from, mitosis. 

Another substrate for the cyclin M/p34 protein kinase may be the carboxy terminal region of the large subunit of RNA polymerase II and this may be related to the switch off of transcription during mitosis (Cisek and Corden, 1989). 

MPF is inactivated (probably by dephosphorylation) following interaction with a 13 kDa polypeptide (homologous to the product of the yeast sue 1 gene). 

Cyclin G1 appears in G1 and is degraded as cells enter S-phase. It interacts with the p34/pl3 complex, releasing pi 3, and reactivating the serine/threonine kinase activity, one substrate of which is cyclin Gl. This leads to initiation of S-phase, though the reactions involved here are unknown (Lee and Nurse, 1988 Murray, 1989). As cyclin Gl is degraded, p34 becomes phosphorylated at tyrosine 15 and can again interact with the accumulating cyclin M. 

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