CONTROL OF MITOTIC EVENTS BY NAP1 AND THE GIN4 KINASE

Little is known about the pathways used by cyclins and cyclin-dependen t kinases to induce the events of the cell cycle. In budding yeast, a protein called Nap1 binds to the mitotic cyclin Clb2, and Nap1 is requ ired for the ability of Clb2 to induce specific mitotic events, but th e role played by Nap1 is unclear. We have used genetic and biochemical approaches to identify additional proteins that function with Nap1 in the control of mitotic events. These approaches have both identified a protein kinase called Gin4 that is required for the ability of Clb2 and Nap1 to promote the switch from polar to isotropic bud growth that normally occurs during mitosis. Gin4 is also required for the ability of Clb2 and Nap1 to promote normal progression through mitosis. The G in4 protein becomes phosphorylated as cells enter mitosis, resulting i n the activation of Gin4 kinase activity, and the phosphorylation of G in4 is dependent upon Nap1 and Clb2 in vivo. Affinity chromatography e xperiments demonstrate that Gin4 binds tightly to Nap1, indicating tha t the functions of these two proteins are closely tied within the cell . These results demonstrate that the activation of Gin4 is under the c ontrol of Clb2 and Nap1, and they provide an important step towards el ucidating the molecular pathways that link cyclin-dependent kinases to the events they control.