Cell Polarity in Saccharomyces cerevisiae Depends on Proper Localization of the Bud9 Landmark Protein by the EKC/KEOPS Complex

Abstract

In diploid Saccharomyces cerevisiae cells, bud-site selection is determined by two cortical landmarks, Bud8p and Bud9p, at the distal and proximal poles, respectively. Their localizations depend on the multigenerational proteins Rax1p/Rax2p. Many genes involved in bud-site selection were identified previously by genome-wide screening of deletion mutants, which identified BUD32 that causes a random budding in diploid cells. Bud32p is an atypical kinase involved in a signaling cascade of Sch9p kinase, the yeast homolog of Akt/PKB, and a component of the EKC/KEOPS (endopeptidase-like, kinase, chromatin-associated/kinase, putative endopeptidase, and other proteins of small size) complex that functions in telomere maintenance and transcriptional regulation. However, its role in bipolar budding has remained unclear. In this report, we show that the Sch9p kinase cascade does not affect bipolar budding but that the EKC/KEOPS complex regulates the localization of Bud9p. The kinase activity of Bud32p, which is essential for the functions of the EKC/KEOPS complex but is not necessary for the Sch9p signaling cascade, is required for bipolar bud-site selection. BUD9 is necessary for random budding in each deletion mutant of EKC/KEOPS components, and RAX2 is genetically upstream of EKC/KEOPS genes for the regulation of bipolar budding. The asymmetric localization of Bud9p was dependent on the complex, but Bud8p and Rax2p were not. We concluded that the EKC/KEOPS complex is specifically involved in the regulation of Bud9p localization downstream of Rax1p/Rax2p.