Myotubularin, a protein tyrosine phosphatase mutated in myotubular myopathy, dephosphorylates the lipid second messenger, phosphatidylinositol 3-phosphate

The lipid second messenger phosphatidylinositol 3-phosphate [PI(3)P] plays a crucial role in intracellular membrane trafficking. We report here that m yotubularin, a protein tyrosine phosphatase required for muscle cell differ entiation, is a potent PI(3)P phosphatase. Recombinant human myotubularin s pecifically dephosphorylates PI(3)P in vitro. Overexpression of a catalytic ally inactive substrate-trapping myotubularin mutant (C375S) in human 293 c ells increases PI(3)P levels relative to that of cells overexpressing the w ild-type enzyme, demonstrating that PI(3)P is a substrate for myotubularin in vivo. In addition, a Saccharomyces cerevisiae strain in which the myotub ularin-like gene (YJR110w) is disrupted also exhibits increased PI(3)P leve ls. Both the recombinant yeast enzyme and a human myotubularin-related prot ein (KIAA0371) are able to dephosphorylate PI(3)P in vitro, suggesting that this activity is intrinsic to all myotubularin family members. Mutations i n the MTM1 gene that cause human myotubular myopathy dramatically reduce th e ability of the phosphatase to dephosphorylate PI(3)P, Our findings provid e evidence that myotubularin exerts its effects during myogenesis by regula ting cellular levels of the inositol lipid PI(3)P.