HKR1 ENCODES A CELL-SURFACE PROTEIN THAT REGULATES BOTH CELL-WALL BETA-GLUCAN SYNTHESIS AND BUDDING PATTERN IN THE YEAST SACCHAROMYCES-CEREVISIAE

We previously isolated the Saccharomyces cerevisiae HKR1 gene that con fers on S. cerevisiae cells resistance to HM-1 killer toxin secreted b y Hansenula mrakii (S. Kasahara, H. Yamada, T. Mio, Y. Shiratori, C. M iyamoto, T. Yabe, T. Nakajima, E. Ichishima, and Y. Furuichi, J. Bacte riol. 176:1488-1499, 1994). HKR1 encodes a type 1 membrane protein tha t contains a calcium-binding consensus sequence (EF hand motif) in the cytoplasmic domain. Although the null mutation of HKR1 is lethal, dis ruption of the 3' part of the coding region, which would result in del etion of the cytoplasmic domain of Hkr1p, did not affect the viability of yeast cells. This partial disruption of HKR1 significantly reduced beta-1,3-glucan synthase activity and the amount of beta-1,3-glucan i n the cell wall and altered the axial budding pattern of haploid cells . Neither chitin synthase activity nor chitin content was significantl y affected in the cells harboring the partially disrupted HKR1 allele. Immunofluorescence microscopy with an antibody raised against Hkr1p e xpressed in Escherichia coli revealed that Hkr1p was predominantly loc alized on the cell surface. The cell surface localization of Hkr1p req uired the N-terminal signal sequence because the C-terminal half of Hk r1p was detected uniformly in the cells. These results demonstrate tha t HKR1 encodes a cell surface protein that regulates both cell wall be ta-glucan synthesis and budding pattern and suggest that bud site asse mbly is somehow related to beta-glucan synthesis in S. cerevisiae.