Eliminating all obstacles: regulated proteolysis in the eukaryotic cell cycle

Eukaryotic cell cycle research took a quantum leap forward approximately ten years ago when it was appreciated that a conserved class of protein kinases propels the cell through the periodic events of mitotic division. The actions of specific yclin-ependent inases (CDKs), so named for their requisite associated cyclin subunits, promote events such as DNA replication and chromosome segregation by the mitotic spindle. The view of the cell cycle as a series of CDK-activated events is only partially complete, however. We now understand that cycle periodicity requires essential degradative processes as well (reviewed in 8). Highly specific proteolytic events initiate cell cycle transitions as well as eliminate cyclically acting proteins at stages when they are no longer required and are possibly deleterious.