Role of K364 next to the active site cysteine in voltage-dependent phosphatase activity of Ci-VSP

Voltage-sensing phosphatase, VSP, consists of the voltage sensor domain (VSD) similar to that of voltage-gated ion channels and the cytoplasmic phosphatase region with remarkable similarity to the phosphatase and tensin homolog deleted on chromosome ten, PTEN. Membrane depolarization activates VSD leading to dephosphorylation of three species of phosphoinositides (PIPs), PI(3,4,5)P₃, PI(4,5)P₂ and PI(3,4)P₂. VSP dephosphorylates 3- and 5- phosphate of PIPs unlike PTEN which shows rigid 3-phosphate specificity. In this study, a bioinformatics search showed that some mammals have VSP orthologs with amino acid diversity in the active center motif, Cx₅R, which is highly conserved among protein tyrosine phosphatases and PTEN-related phosphatases; lysine next to the active site cysteine in the Cx₅R motif was substituted for methionine in VSP orthologs of Tasmanian devil, koala and prairie deer mouse, and leucine in opossum. Since lysine at the corresponding site in PTEN is known to be critical for enzyme activities, we attempted to address significance of amino acid diversity among VSP orthologs at this site. K364 was changed to different amino acids in sea squirt VSP (Ci-VSP) and voltage-dependent phosphatase activity in Xenopus oocyte was studied using fluorescent probes for PI(4,5)P₂ and PI(3,4)P₂. All mutants retained both 5-phosphatase and 3-phosphatase activity, indicating that lysine at this site is dispensable for 3-phosphatase activity unlike PTEN. Notably, K364M mutant showed increased activity both of 5-phosphatase and 3-phosphatase as compared with the wild type (WT). It also showed slower kinetics of voltage sensor motion. Malachite green assay of K364M mutant did not show significant difference of phosphatase activity from WT, suggesting tighter interaction between substrate binding and voltage sensing. Mutation corresponding to K364M in the zebrafish VSP led to enhanced voltage-dependent dephosphorylation of PI(4,5)P₂. Further studies will provide clues to understanding of substrate preference in PIPs phosphatases as well as to customization of molecular tool. (300 words).