Acts as an adapter for the phosphatase MTMR2 to regulate MTMR2 catalytic activity and subcellular location (PubMed:12668758). May function as a guanine nucleotide exchange factor (GEF) activating RAB28 (PubMed:20937701). Promotes the exchange of GDP to GTP, converting inactive GDP-bound Rab proteins into their active GTP-bound form (PubMed:20937701). Inhibits myoblast differentiation in vitro and induces oncogenic transformation in fibroblasts (PubMed:9537414). Heterodimer with lipid phosphatase MTMR2 (PubMed:12668758). Interacts with KMT2A/MLL1 (via SET domain) (PubMed:9537414). Interacts with SUV39H1 (PubMed:10848615). The C-terminal domain mediates interaction with MTMR2. The disease is caused by variants affecting the gene represented in this entry. 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 (PubMed:9537414). The pocket is however sufficiently preserved to bind phosphorylated substrates, and maybe protect them from phosphatases.