THE HIGHLY CONSERVED SKB1 GENE ENCODES A PROTEIN THAT INTERACTS WITH SHK1, A FISSION YEAST STE20 PAK HOMOLOG/

The Shk1 protein kinase, a homolog of Saccharomyces cerevisiae Ste20 a nd mammalian p21(Cdc42/Rac)-activated kinases, is an essential compone nt of a Ras- and Cdc42-dependent signaling cascade required for cell v iability, normal morphology, and mitogen-activated protein kinase-medi ated sexual responses in the fission yeast, Schizosaccharomyces pombe, To identify S. pombe proteins that modulate or mediate Shk1 functions , we conducted a two-hybrid screen for Shk1-interacting proteins, One of the genes identified as a result of this screen was skb1, We show t hat Skb1 interacts with a region of the N-terminal regulatory domain o f Shk1 distinct from that to which Cdc42 binds, and that Shk1, Cdc42, and Skb1 are able to form a ternary complex in vivo. S. pombe cells ca rrying an skb1 null mutation are less elongate in morphology than mild -type cells and exhibit a moderate growth defect, The morphology defec t of the skb1 deletion mutant is suppressed by overexpression of Shk1, Overexpression of Skb1 causes wild-type S. pombe cells to become hype relongated, Additional genetic analyses described herein suggest that Skb1 is a component of the morphology control branch of the Ras signal ing cascade in S. pombe and that it positively modulates Shk1 function , Homologs of Skb1 are encoded by open reading frames in the genomes o f S. cerevisiae and Caenorhabditis elegans and by an uncharacterized h uman cDNA sequence, Thus, skb1 mag be the first well-characterized mem ber of a highly conserved family of genes encoding potential p21(Cdc42 /Rac)-activated kinase regulators.