CONTROL OF MITOTIC EVENTS BY THE CDC42 GTPASE, THE CLB2 CYCLIN AND A MEMBER OF THE PAK KINASE FAMILY

Background: Cyclins and cyclin-dependent kinases induce and coordinate the events of the cell cycle, although the mechanisms by which they d o so remain largely unknown. In budding yeast, a pathway used by the C lb2 cyclin to control bud growth during mitosis provides a good model system in which to understand how cyclin-dependent kinases control cel l-cycle events. In this pathway, Clb2 initiates a series of events tha t lead to the mitosis-specific activation of the Gin4 protein kinase, A protein called Nap1 is required in vivo for the activation of Gin4, and is able to bind to both Gin4 and Clb2. We have used a simple genet ic screen to identify additional proteins that function in this pathwa y.Results: We have found that the Cdc42 GTPase and a member of the PAK kinase family called Cla4 both function in the pathway used by Clb2 t o control bud growth during mitosis. Cdc42 and Cla4 interact genetical ly with Gin4, and Nap1, and both are required in vivo for the mitosis- specific activation of the Gin4 kinase, Furthermore, Cla4 undergoes a dramatic hyperphosphorylation in response to the combined activity of Nap1, the Clb2-Cdc28 kinase complex, and the GTP-bound form of Cdc42. Evidence is presented which suggests that the hyperphosphorylated form of Cla4 is responsible for relaying the signal to activate Gin4. Conc lusions: Previous studies have suggested that cyclin-dependent kinases control the cell cycle by directly phosphorylating proteins involved in specific events, such as nuclear lamins, microtubule-associated pro teins and histones. In contrast, our results demonstrate that the Clb2 -Cdc28 cyclin-dependent kinase complex controls specific cell-cycle ev ents through a pathway that involves a GTPase and at least two differe nt kinases, This suggests that cyclin-dependent kinases may control ma ny cell-cycle events through GTPase-linked signaling pathways that res emble the intricate signaling pathways known to control many other cel lular events.