Acts as an adapter for the phosphatase MTMR2 to regulate MTMR2 catalytic activity and subcellular location. May function as a guanine nucleotide exchange factor (GEF) activating RAB28. Promotes the exchange of GDP to GTP, converting inactive GDP-bound Rab proteins into their active GTP-bound form. Inhibits myoblast differentiation in vitro and induces oncogenic transformation in fibroblasts. Heterodimer with lipid phosphatase MTMR2. Interacts with KMT2A/MLL1 (via SET domain). Interacts with SUV39H1. The C-terminal domain mediates interaction with MTMR2. Belongs to the protein-tyrosine phosphatase family. Non-receptor class myotubularin subfamily. Although it belongs to the non-receptor class myotubularin subfamily, lacks the conserved active site cysteine residue at position 1422 in the dsPTPase catalytic loop and does not have phosphatase activity (By similarity). The pocket is however sufficiently preserved to bind phosphorylated substrates, and maybe protect them from phosphatases. Truncated N-terminus. Extended N-terminus.