Polarized Growth Controls Cell Shape and Bipolar Bud Site Selection in Saccharomyces cerevisiae

Abstract

We examined the relationship between polarized growth and division site selection, two fundamental processes important for proper development of eukaryotes. Diploid Saccharomyces cerevisiaecells exhibit an ellipsoidal shape and a specific division pattern (a bipolar budding pattern). We found that the polarity genesSPA2, PEA2, BUD6, andBNI1 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 apical growth. Examination of distribution of the polarized secretion marker Sec4 demonstrates that spa2Δ, pea2Δ,bud6Δ, and bni1Δ mutants fail to concentrate Sec4 at the bud tip during apical growth and at the division site during repolarization just prior to cytokinesis. Moreover, cell surface expansion is not confined to the distal tip of the bud in these mutants. In addition, we found that the p21-activated kinase homologue Ste20 is also important for both apical growth and bipolar bud site selection. We further examined how the duration of polarized growth affects bipolar bud site selection by using mutations in cell cycle regulators that control the timing of growth phases. Thegrr1Δ mutation enhances apical growth by stabilizing G₁ cyclins and increases the distal-pole budding in diploids. Prolonging polarized growth phases by disrupting the G₂/M cyclin gene CLB2 enhances the accuracy of bud site selection in wild-type, spa2Δ, andste20Δ cells, whereas shortening the polarized growth phases by deleting SWE1 decreases the fidelity of bipolar budding. This study reports the identification of components required for apical growth and demonstrates the critical role of polarized growth in bipolar bud site selection. We propose that apical growth and repolarization at the site of cytokinesis are crucial for establishing spatial cues used by diploid yeast cells to position division planes.

ACKNOWLEDGMENTS

We thank E. Leberer and B. Santos for generous gifts of plasmids and strains. We also thank P. Novick for kindly providing the Sec4 monoclonal antibody. We thank C. Costigan, B. Manning, G. Michaud, B. Santos, and S. Vidan for critical comments on the manuscript. We also thank the G. S. Roeder laboratory for use of the microscope and the charge-coupled device camera.

This research was supported by National Institute of Health grant GM36494.