MOB1, AN ESSENTIAL YEAST GENE REQUIRED FOR COMPLETION OF MITOSIS AND MAINTENANCE OF PLOIDY

Mob1p is an essential Saccharomyces cerevisiae protein, identified fro m a two-hybrid screen, that binds Mps1p, a protein kinase essential fo r spindle pole body duplication and mitotic checkpoint regulation. Mob 1p contains no known structural motifs; however MOB1 is a member of a conserved gene family and shares sequence similarity with a nonessenti al yeast gene, MOB2. Mob1p is a phosphoprotein in vivo and a substrate for the Mps1p kinase in vitro. Conditional alleles of MOB1 cause a la te nuclear division arrest at restrictive temperature. MOB1 exhibits g enetic interaction with three other yeast genes required for the compl etion of mitosis, LTE1, CDC5 and CDC15 (the latter two encode essentia l protein kinases). Most haploid mutant mob1 strains also display a co mplete increase in ploidy at permissive temperature. The mechanism for the increase in ploidy may occur through MPS1 function. One mob1 stra in, which maintains stable haploidy at both permissive and restrictive temperature, diploidizes at permissive temperature when combined with the mps1-1 mutation. Strains containing mob2 Delta also display a com plete increase in ploidy when combined with the mps1-1 mutation. Perha ps in addition to, or as part of, its essential function in late mitos is, MOB1 is required for a cell cycle reset function necessary for the initiation of the spindle pole body duplication.