CLONING AND CHARACTERIZATION OF SHK2 A GENE ENCODING A NOVEL P21-ACTIVATED PROTEIN-KINASE FROM FISSION YEAST

We describe the characterization of a novel gene, shk2, encoding a sec ond p21(cdc42/rac)-activated protein kinase (PAK) homolog in fission y east. Like other known PAKs, Shk2 binds to Cdc42 in vivo and in vitro. While overexpression of either shk2 or cdc42 alone does not impair gr owth of wild type fission yeast cells, cooverexpression of the two gen es is toxic and leads to highly aberrant cell morphology, providing ev idence for functional interaction between Cdc42 and Shk2 proteins in v ivo. Fission yeast shk2 null mutants are viable and exhibit no obvious phenotypic defects, Overexpression of shk2 restores viability and nor mal morphology but not full mating competence to fission yeast cells c arrying a shk1 null mutation. Additional genetic data suggest that Shk 2, like Cdc42 and Shk1, participates in Ras-dependent morphological co ntrol and mating response pathways in fission yeast. We also show that overexpression of byr2, a gene encoding a Stell/MAPK kinase kinase ho molog, suppresses the mating defect of cells partially defective for s hk1 function, providing evidence of a link between PAKs and mitogen-ac tivated protein kinase signaling in fission yeast. Taken together, our results suggest that Shk2 is partially overlapping in function with S hk1, with Shk1 being the dominant protein in function.