Negative regulation of mitosis in fission yeast by the Shk1-interacting protein Skb1 and its human homolog, Skb1Hs

We previously provided evidence that the protein encoded by the highly cons erved skb1 gene is a putative regulator of Shk1, a p21(Cdc42/Rac)-activated kinase (PAR) homolog in the fission yeast Schizosaccharomyces pombe. skb1 null mutants are viable and competent for mating but less elongate than wil d-type S. pombe cells, whereas cells that overexpress skb1 are hyperelongat ed. These phenotypes suggest a possible role for Skb1 as a mitotic inhibito r. Here we show genetic interactions of both skb1 and shk1 with genes encod ing key mitotic regulators in S. pombe. Our results indicate that Skb1 nega tively regulates mitosis by a mechanism that is independent of the Cdc2-act ivating phosphatase Cdc25 but that is at least partially dependent on Shk1 and the Cdc2 inhibitory kinase Wee1. We provide biochemical evidence for as sociation of Skb1 and Shk1 with Cdc2 in S. pombe, suggesting that Skb1 and Shk1 inhibit mitosis through interaction with the Cdc2 complex, rather than by an indirect mechanism. These results provide evidence of a previously u ndescribed role for PAR-related protein kinases as mitotic inhibitors. We a lso describe the cloning of a human homolog of skb1, SKB1Hs, and show that it can functionally replace skb1 in S. pombe. Thus, the molecular functions of Skb1-related proteins have likely been substantially conserved through evolution.