The cell
polarity factor, Tea1, is a potential downstream target of the Ste20/PAK
homolog, Shk1, in the fission yeast, Schizosaccharomyces pombe.
Peirong Yang, HyeWon Kim, Stevan Marcus
Dept. of Molecular Genetics, UT M.D. Anderson Cancer Center, 1515 Holcombe
Blvd., Houston, TX 77030, USA (smarcus@mdacc.tmc.edu)
The Ste20/PAK homolog, Shk1, is an essential regulator of polarized growth in the fission yeast, Schizosaccharomyces pombe. Substrate targets of the Shk1 kinase have not previously described. Here we investigated whether Shk1 interacts with Tea1, a kelch repeat protein that, like Shk1, regulates cell polarity in S. pombe. We show that a null mutation in the tea1 gene suppresses the Shk1 hyperactivity-induced lethal phenotype caused by loss of the Shk1 inhibitor, Skb15, thus implicating Tea1 as a potential Shk1 effector. All phenotypes associated with Skb15 loss, including defects in actin cytoskeletal organization, chromosome segregation, and cytokinesis, are suppressed by tea1Δ, suggesting that Tea1 is a potential mediator of multiple Shk1 functions. Consistent with a role for Tea1 as a downstream target of Shk1, we show that the Tea1 protein is phosphorylated in S. pombe cells and that its level of phosphorylation is greatly reduced in cells defective in Shk1 function. We show further that bacterially expressed Tea1 protein is directly phosphorylated by recombinant Shk1 kinase in vitro. S. pombe cells carrying a weak hypomorphic allele of shk1 together with a tea1Δ mutation exhibit a cytokinesis defective phenotype that is significantly more severe than that observed in the respective single mutants, providing evidence that Shk1 and Tea1 cooperate to regulate cytokinesis. Our results strongly implicate Tea1 as a potential downstream target of the Shk1 kinase.