Rot1 plays an antagonistic role to Clb2 in actin cytoskeleton dynamics throughout the cell cycle

ROT1 is an essential gene whose inactivation causes defects in cell cycle progression and morphogenesis in budding yeast. Rot1 affects the actin cytoskeleton during the cell cycle at two levels. First, it is required for the maintenance of apical growth during bud growth. Second, Rot1 is necessary to polarize actin cytoskeleton to the neck region at the end of mitosis; because of this defect, rot1 cells do not properly form a septum to complete cell division. The inability to polarize the actin cytoskeleton at the end of mitosis is not due to a defect in the recruitment of the polarisome scaffold protein Spa2 or the actin cytoskeleton regulators Cdc42 and Cdc24 in the neck region. Previous results indicate a connection between Rot1 and the cyclin Clb2. In fact, overexpression of CLB2 is toxic when ROT1 is partially inactivated, and reciprocally, deletion of CLB2 suppresses the lethality of the rot1 mutant, which indicates a functional antagonism between Clb2 and Rot1. Several genetic interactions suggest a link between Rot1 and the ubiquitin-proteasome system and we show that the Clb2 cyclin is not properly degraded in rot1 cells.