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Sinsheimer Laboratories, Department of Biology, University of California, Santa Cruz, California 95064
Little is known about the pathways used by
cyclins and cyclin-dependent 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 required for the ability of Clb2 to induce specific mitotic events, but the 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 Gin4 protein becomes phosphorylated as
cells enter mitosis, resulting in the activation of Gin4 kinase activity, and the phosphorylation of Gin4 is dependent upon Nap1 and Clb2 in vivo. Affinity chromatography experiments demonstrate that Gin4 binds tightly
to Nap1, indicating that the functions of these two proteins are closely tied within the cell. These results demonstrate that the activation of Gin4 is under the control
of Clb2 and Nap1, and they provide an important step
towards elucidating the molecular pathways that link
cyclin-dependent kinases to the events they control.
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