Cyclin structure, function and destruction

Mitotic cyclins A and B bind tightly to p34cdc2, endowing the complex with protein kinase activity. The destruction of cyclins by tightly regulated, specific proteolysis is required to turn off these kinases and permit the return to interphase. In the rapid cleavage cell cycles of early amphibian and molluscan embryos, the cyclin-specific protease system is active during a brief window that opens a minute or two before the metaphase-anaphase transition and closes about 5 minutes later. But in cultured fibroblasts, destruction of B-type cyclins remains active for several hours after the end of mitosis, and does not turn off until the end of G1, when S-phase starts. This mechanism plays a major role in keeping the levels of B-type cyclins low during G1 phase. We will discuss how many other targets exist for this cell-cycle phase specific proteoiysis system, as well as the mechanism and control of proteolysis.