Polo kinase: the choreographer of the mitotic stage?

The regulation of protein function through phosphorylation is fundamental in controlling cell cycle progression. To date, most attention has focused on the cyclin-dependent protein kinases (cdks) 1 (for review see reference 21). However, whereas the p34^(cdc)-cyclin B complex appears to regulate the mitotic "state" and in this way changes the overall organization of the cell, members of another conserved serine/threonine kinase family appears to be able to control the dynamics of cellular architecture. These are the polo-like kinases (plks) which orchestrate several mitotic events including the formation of the bipolar spindle, and at least in some organisms, the process of cytokinesis. It appears that in some of its roles the plk cooperates with p34^(cdc2) and indeed recent work (15) has suggested that one plk can help maintain the mitotic state by phosphorylating the cdc25 phosphatase that activates p34^(cdc2).