THE SLT2 (MPK1) MAP KINASE HOMOLOG IS INVOLVED IN POLARIZED CELL-GROWTH IN SACCHAROMYCES-CEREVISIAE

Bud emergence, spindle pole body duplication and DNA replication are a ll dependent on the activation of the CDC28 protein kinase at the Star t point in the G1 phase of the cell cycle. Bud emergence requires pola rization of the cytoskeleton and secretory vesicles to a specific site on the cell surface. Cdc28p activated by G1-cyclins triggers polariza tion of actin to the site of bud emergence and favors apical bud growt h (Lew, D. J., and S. I. Reed. 1993. J. Cell Biol. 120:1305-1320). We isolated slt2-1 as a mutation that enhances the division defect of cdc 28 mutants with defects at Start. Slt2p(Mpk1p) is a member of the MAP kinase family (Lee, K. S., K. Irie, Y. Gotoh, Y. Watanabe, H. Araki, E . Nishida, K. Matsumoto, and D. E. Levin. 1993. Mol. Cell. Biol. 13:30 673075). We show that slt2 mutants exhibit phenotypes similar to those shown by mutants of the yeast actin cytoskeleton, including delocaliz ation of chitin deposition and of actin cortical spots and the accumul ation of secretory pathway membranes and vesicles. Furthermore, slt2=H IS3 act1-1 and slt2=HIS3 myo2-66 double mutants are inviable. We sugge st that Slt2p functions downstream or in parallel with Cdc28p in promo ting bud formation and apical growth.