Lipid phosphatase that has high specificity for phosphatidylinositol 3-phosphate and has no activity with phosphatidylinositol 4-phosphate, phosphatidylinositol (4,5)-bisphosphate and phosphatidylinositol (3,4,5)-trisphosphate (PubMed:12217958). Activity with phosphatidylinositol (3,5)-bisphosphate is controversial; it has been shown for the human ortholog (By similarity). In contrast, PubMed:12217958 find no activity with this substrate. a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol-3,5-bisphosphate) + H2O = a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol-5-phosphate) + phosphate a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol-3-phosphate) + H2O = a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol) + phosphate 1,2-dioctanoyl-sn-glycero-3-phospho-(1-D-myo-inositol-3-phosphate) + H2O = 1,2-dioctanoyl-sn-glycero-3-phospho-(1D-myo-inositol) + phosphate Homodimer. Widely expressed. Detected in skeletal muscle, heart, lung, liver and brain. The C-terminal region is required for dimerization. Belongs to the protein-tyrosine phosphatase family. Non-receptor class myotubularin subfamily.