The highly conserved skb1 gene encodes a protein that interacts with Shkl, a fission yeast Ste20/PAK homolog

The Shkl protein kinase, a homolog of Saccharomyces cerevisiae Ste20 and mammalian p21^Cdc42/Rac-activated kinases, is an essential component of a Ras- and Cdc42-dependent signaling cascade required for cell viability, normal morphology, and mitogen-activated protein kinase-mediated sexual responses in the fission yeast, Schizosaccharomyces pombe. To identify S. pombe proteins that modulate or mediate Shkl functions, we conducted a two-hybrid screen for Shkl-interacting proteins. One of the genes identified as a result of this screen was skbl. We show that Skbl interacts with a region of the N-terminal regulatory domain of Shkl distinct from that to which Cdc42 binds, and that Shkl, Cdc42, and Skbl are able to form a ternary complex in vivo. S. pombe cells carrying an skbl null mutation are less elongate in morphology than wild-type cells and exhibit a moderate growth defect. The morphology defect of the skbl deletion mutant is suppressed by overexpression of Shkl. Overexpression of Skbl causes wild-type S. pombe cells to become hyperelongated. Additional genetic analyses described herein suggest that Skbl is a component of the morphology control branch of the Ras signaling cascade in S. pombe and that it positively modulates Shkl function. Homologs of Skbl are encoded by open reading frames in the genomes of S. cerevisiae and Caenorhabditis elegans and by an uncharacterized human cDNA sequence. Thus, skbl may be the first well-characterized member of a highly conserved family of genes encoding potential p21^Cdc42/Rac-activated kinase regulators.