Myotubularin, a phosphatase deficient in myotubular myopathy, acts on phosphatidylinositol 3-kinase and phosphatidylinositol 3-phosphate pathway.
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.
Blondeau
F
F
Institut de Génétique et de Biologie Moléculaire et Cellulaire, CNRS/INSERM/ULP, 1 rue Laurent Fries, BP 163, 67404 Illkirch Cedex, CU de Strasbourg, France.
Laporte
J
J
Bodin
S
S
Superti-Furga
G
G
Payrastre
B
B
Mandel
J L
JL
D000818:Animals; D002460:Cell Line; D015536:Down-Regulation; D006801:Humans; D020914:Myopathies, Structural, Congenital / Q000235:genetics*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D018129:Phosphatidylinositol Phosphates / Q000378:metabolism*; D011233:Precipitin Tests; D017027:Protein Tyrosine Phosphatases / Q000235:genetics* / Q000378:metabolism; D054558:Protein Tyrosine Phosphatases, Non-Receptor; D012568:Schizosaccharomyces / Q000201:enzymology / Q000235:genetics / Q000378:metabolism; D014162:Transfection
D019869:Phosphatidylinositol 3-Kinases; D018129:Phosphatidylinositol Phosphates; D017027:Protein Tyrosine Phosphatases; D054558:Protein Tyrosine Phosphatases, Non-Receptor; C055525:phosphatidylinositol 3-phosphate; C099588:myotubularin