CDC42P GTPASE IS INVOLVED IN CONTROLLING POLARIZED CELL-GROWTH IN SCHIZOSACCHAROMYCES-POMBE

Cdc42p is a highly conserved low-molecular-weight GTPase that is invol ved in controlling cellular morphogenesis. We have isolated the Cdc42p homolog from the fission yeast Schizosaccharomyces pombe by its abili ty to complement the Saccharomyces cerevisiae cdc42-1(ts) mutation. S. pombe Cdc42p is 85% identical in predicted amino acid sequence by S. cerevisiae Cdc42p and 83% identical to the human Cdc-42p homolog. The Cdc42p protein fractionates to both soluble and particulate fractions, suggesting that it exists in two cellular pools. We have disrupted th e cdc42(+) gene and shown that it is essential for growth. The cdc42 n ull phenotype is an arrest as small, round, dense cells. In addition, we have generated three site-specific mutations, G12V, Q61L, and D118A , in the Cdc42p GTP-binding domains that correspond to dominant-lethal mutations in S. cerevisiae CDC42. In contrast to the S. cerevisiae cd c42 mutations, the S. pombe cdc42 mutants alleles were not lethal when overexpressed. However, the cdc42 mutants did exhibit an abnormal mor phological phenotype of large, misshapen cells, suggesting the S. pomb e Cdc42p is involved in controlling polarized cell growth.