Phosphatase that acts on lipids with a phosphoinositol headgroup. Dephosphorylates phosphatidylinositol 3-phosphate (PtdIns(3)P) and phosphatidylinositol 3,5-bisphosphate. Binds with high affinity to phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P2) but also to phosphatidylinositol 3-phosphate (PtdIns(3)P), phosphatidylinositol 4-phosphate (PtdIns(4)P), and phosphatidylinositol 5-phosphate (PtdIns(5)P), phosphatidic acid and phosphatidylserine (By similarity). Negatively regulates ER-Golgi protein transport (By similarity). Probably in association with MTMR9, plays a role in the late stages of macropinocytosis by dephosphorylating phosphatidylinositol 3-phosphate in membrane ruffles (PubMed:24591580). Acts as a negative regulator of KCNN4/KCa3.1 channel activity in CD4(+) T-cells possibly by decreasing intracellular levels of phosphatidylinositol 3-phosphate. Negatively regulates proliferation of reactivated CD4(+) T-cells. In complex with MTMR9, negatively regulates DNA damage-induced apoptosis. The formation of the MTMR6-MTMR9 complex stabilizes both MTMR6 and MTMR9 protein levels (By similarity). 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-(1D-myo-inositol-3,5-bisphosphate) + H2O = 1,2-dioctanoyl-sn-glycero-3-phospho-(1D-myo-inositol-5-phosphate) + 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 Allosterically activated by phosphatidylserine and/or phosphatidylinositol 4-phosphate (PtdIns(4)P), and phosphatidylinositol 5-phosphate (PtdIns(5)P) (By similarity). Interaction with MTMR9 increases catalytic activity towards phosphatidylinositol 3,5-bisphosphate (By similarity). Homodimer (By similarity). Heterodimer (via C-terminus) with MTMR9 (via C-terminus) (PubMed:23188820, PubMed:12890864). Interacts with ALKBH4 (By similarity). Interacts with KCNN4 (By similarity). Interacts (via GRAM domain) with RAB1B (in GDP-bound form); the interaction regulates MTMR6 recruitment to the endoplasmic reticulum-Golgi intermediate compartment (PubMed:23188820). Localizes to ruffles during EGF-induced macropinocytosis (PubMed:24591580). Colocalizes with MTMR9 to the perinuclear region (By similarity). Partially localizes to the endoplasmic reticulum (By similarity). Co-localizes with RAB1B to the endoplasmic reticulum-Golgi intermediate compartment and to the peri-Golgi region (By similarity). Isoform 1: Ubiquitously expressed including in heart, brain, spleen, lung, liver, muscle, kidney and testis (at protein level) (PubMed:23188820). Isoform 2: Expressed in testis (at protein level) (PubMed:23188820). The GRAM domain is required for cell membrane localization. The C-terminus domain (aa 502-617) mediates interaction with MTMR9. Belongs to the protein-tyrosine phosphatase family. Non-receptor class myotubularin subfamily.