Polarized growth controls cell shape and bipolar bud site selection in Saccharomyces cerevisiae

We examined the relationship between polarized growth and division site sel ection, two fundamental processes important for proper development of eukar yotes. Diploid Saccharomyces cerevisiae cells exhibit an ellipsoidal shape and a specific division pattern (a bipolar budding pattern). We found that the polarity genes SPA2, PEA2, BUD6, and BNI1 participate in a crucial step of bud morphogenesis, apical growth. Deleting these genes results in round cells and diminishes bud elongation in mutants that exhibit pronounced api cal growth. Examination of distribution of the polarized secretion marker S ec4 demonstrates that spa2 Delta, pea2 Delta, bud6 Delta, and bni1 Delta mu tants fail to concentrate Sec4 at the bud tip during apical growth and at t he division site during repolarization just prior to cytokinesis. Moreover, cell surface expansion is not confined to the distal tip of the bud in the se mutants. In addition, we found that the p21-activated kinase homologue S te20 is also important for both apical growth and bipolar bud site selectio n. We further examined how the duration of polarized growth affects bipolar bud site selection by using mutations in cell cycle regulators that contro l the timing of growth phases. The grr1 Delta mutation enhances apical grow th by stabilizing G(1) cyclins and increases the distal-pole budding in dip loids. Prolonging polarized growth phases by disrupting the G(2)/M cyclin g ene CLB2 enhances the accuracy of bud site selection in wild-type, spa2 Del ta, and ste20 Delta cells, whereas shortening the polarized growth phases b y deleting SWE1 decreases the fidelity of bipolar budding. This study repor ts the identification of components required for apical growth and demonstr ates the critical role of polarized growth in bipolar bud site selection. W e propose that apical growth and repolarization at the site of cytokinesis are crucial for establishing spatial cues used by diploid yeast cells to po sition division planes.