Cla4p, a Saccharomyces cerevisiae Cdc42p-activated kinase involved in cytokinesis, is activated at mitosis

Yeasts have three functionally redundant G(1) cyclins required for cell cycle progression through G(1). Mutations in GIN4 and CLA4 were isolated in a screen for mutants that are inviable with deletions in the G(1) cyclins CLN1 and CLN2. cln1 cln2 cla4 and cln1 cln2 gin4 cells arrest with a cytokinesis defect; this defect was efficiently rescued by CLN1 or CLN2 expression. GIN4 encodes a protein with strong homology to the Snf1p serine/threonine kinase. Cla4p is homologous to mammalian p21-activated kinases (PAKs) (kinases activated by the rho-class GTPase Rac dr Cdc42). We developed a kinase assay for Cla4p. Cla4p kinase was activated in vivo by the GTP-bound form of Cdc42p. The specific activity of Cla4p was cell cycle regulated, peaking near mitosis. Deletion of the Cla4p pleckstrin domain diminished kinase activity nearly threefold and eliminated in vivo activity. Deletion of the Cla4p Cdc42-binding domain increased kinase activity nearly threefold, but the mutant only weakly rescued cla4 function in vivo. This suggests that kinase activity alone is not sufficient for full function in vivo. Deletion of the Cdc42-binding domain also altered the cell cycle regulation of kinase activity. Instead of peaking at mitosis, the mutant kinase activity exhibited reduced cell cycle regulation and peaked at the G,IS border. Cla4p kinase activity was not reduced by mutational inactivation of gin4, suggesting that Gin4p may be downstream or parallel to Cla4p in the regulation of cytokinesis.