The structure, function and destruction of mitotic cyclins

The periodic synthesis and destruction of B-typcc>clins is required for the properly timed appearance and disappearance of Cdc2 activity at the beginning and end of mitosis. Five distinct B-type cyclins are known in frogs, whose functions arc poorly defined. We have created a nullizygous cyclin B2 mouse, which appears to be completely normal. Cyclin B3 is known in chick, fly, nematode worm and frog but no mammalian clones are known so far. We are in the process of knocking out the mouse cyclin B l gene to test the degree of redundancy in this family. We are aiso studying the patterns of expression of the many frog B-type cyclms during early embryonic development. At the metaphase-anaphase transition in mitosis, the B-type cyclms are degraded by the ubiquitin-proteasome pathway, b> a pathway that requires the 9-residue 'destruction box' near their N-terminus. Strong expression of the N-terminal domain of the S. pombc B-lype cyclin Cdc 13 inhibits cell cycle progression and colony formation. N-terminal fragments lacking lysine residues arc much less toxic than their wild-type counterparts, although they still inhibit growth when expressed in cells carrying mutations in components of the APC/cyclosome. from which we deduce that their toxicity stems partly from general competition for ubiquitin and other components of the proteolysis machinery, and partly from competition for the 'destruction box recognition element' that we seek Whereas the destruction pathway is rapidly turned off after the end of mitosis in the rapid embryonic cell cycles, in somatic cells, cyclin proleolysis remains active throughout G1. We are trying to understand the mechanism and control of this process.