Myotubularin, a phosphatase deficient in myotubular myopathy, acts on phosphatidylinositol 3-kinase and phosphatidylinositol 3-phosphate pathway

Hum Mol Genet. 2000 Sep 22;9(15):2223-9. doi: 10.1093/oxfordjournals.hmg.a018913.


Myotubular myopathy (MTM1) is an X-linked disease, characterized by severe neonatal hypotonia and generalized muscle weakness, with pathological features suggesting an impairment in maturation of muscle fibres. The MTM1 gene encodes a protein (myotubularin) with a phosphotyrosine phosphatase consensus. It defines a family of at least nine genes in man, including the antiphosphatase hMTMR5/Sbf1 and hMTMR2, recently found mutated in a recessive form of Charcot-Marie-Tooth disease. Myotubularin shows a dual specificity protein phosphatase activity in vitro. We have performed an in vivo test of tyrosine phosphatase activity in Schizosaccharomyces pombe, indicating that myotubularin does not have a broad specificity tyrosine phosphatase activity. Expression of active human myotubularin inhibited growth of S.pombe and induced a vacuolar phenotype similar to that of mutants of the vacuolar protein sorting (VPS) pathway and notably of mutants of VPS34, a phosphatidylinositol 3-kinase (PI3K). In S.pombe cells deleted for the endogenous MTM homologous gene, expression of human myotubularin decreased the level of phosphatidylinositol 3-phosphate (PI3P). We have created a substrate trap mutant which shows relocalization to plasma membrane projections (spikes) in HeLa cells and was inactive in the S.pombe assay. This mutant, but not the wild-type or a phosphatase site mutant, was able to immunoprecipitate a VPS34 kinase activity. Wild-type myotubularin was also able to directly dephosphorylate PI3P and PI4P in vitro. Myotubularin may thus decrease PI3P levels by down-regulating PI3K activity and by directly degrading PI3P.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Cell Line
  • Down-Regulation
  • Humans
  • Myopathies, Structural, Congenital / genetics*
  • Phosphatidylinositol 3-Kinases / metabolism*
  • Phosphatidylinositol Phosphates / metabolism*
  • Precipitin Tests
  • Protein Tyrosine Phosphatases / genetics*
  • Protein Tyrosine Phosphatases / metabolism
  • Protein Tyrosine Phosphatases, Non-Receptor
  • Schizosaccharomyces / enzymology
  • Schizosaccharomyces / genetics
  • Schizosaccharomyces / metabolism
  • Transfection


  • Phosphatidylinositol Phosphates
  • phosphatidylinositol 3-phosphate
  • Protein Tyrosine Phosphatases
  • Protein Tyrosine Phosphatases, Non-Receptor
  • myotubularin