NAP1 ACTS WITH CLB2 TO PERFORM MITOTIC FUNCTIONS AND TO SUPPRESS POLAR BUD GROWTH IN BUDDING YEAST

NAP1 is a 60-kD protein that interacts specifically with mitotic cycli ns in budding yeast and frogs. We have examined the ability of the yea st mitotic cyclin Clb2 to function in cells that lack NAP1. Our result s demonstrate that Clb2 is unable to carry out its full range of funct ions without NAP1, even though Clb2/p34(CDC28)-associated kinase activ ity rises to normal levels. In the absence of NAP1, Clb2 is unable to efficiently induce mitotic events, and cells undergo a prolonged delay at the short spindle stage with normal levels of Clb2/p34(CDC28) kina se activity. NAP1 is also required for the ability of Clb2 to induce t he switch from polar to isotropic bud growth. As a result, polar bud g rowth continues during mitosis, giving rise to highly elongated cells. Our experiments also suggest that NAP1 is required for the ability of the Clb2/p34(CDC28) kinase nase complex to amplify its own production , and that NAP1 plays a role in regulation of microtubule dynamics dur ing mitosis. Together, these results demonstrate that NAP1 is required for the normal function of the activated Clb2/p34(CDC28) kinase compl ex, and provide a step towards understanding how cyclin-dependent kina se complexes induce specific events during the cell cycle.