The Cdc42p GTPase is involved in a G(2)/M morphogenetic checkpoint regulating the apical-isotropic switch and nuclear division in yeast

The Cdc42p GTPase is involved in the signal transduction cascades controlli ng bud emergence and polarized cell growth in S. cerevisiae. Cells expressi ng the cdc42(V44A) effector domain mutant allele displayed morphological de fects of highly elongated and multielongated budded cells indicative of a d efect in the apicalisotropic switch in bud growth. In addition, these cells contained one, two, or multiple nuclei indicative of a G(2)/M delay in nuc lear division and also a defect in cytokinesis and/or cell separation. Acti n and chitin were delocalized, and septin ring structure was aberrant and p artially delocalized to the tips of elongated Cdc42(V44A) cells; however, C dc42(V44A)p localization was normal. Two-hybrid protein analyses showed tha t the V44A mutation interfered with Cdc42p's interactions with Cla4p, a p21 (Cdc42/Rac)-activated kinase (PAK)-like kinase, and the novel effecters Gic 1p and Gic2p, but not with the Ste20p or Skm1p PAK-like kinases, the Bni1p formin, or the Iqg1p IQGAP homolog. Furthermore, the CdC42V44A morphologica l defects were suppressed by deletion of the Swe1p cyclin-dependent kinase inhibitory kinase and by overexpression of Cla ip, Ste20p, the Cdc12 septin protein, or the guanine nucleotide exchange factor Cdc24p, In sum, these r esults suggest that proper Cdc42p function is essential for timely progress ion through the apical-isotropic switch and G(2)/M transition and that Cdc4 2(V44A)p differentially interacts with a number of effecters and regulators .