Lipid phosphatase which dephosphorylates phosphatidylglycerophosphate (PGP) to phosphatidylglycerol (PG) (By similarity). PGP is an essential intermediate in the biosynthetic pathway of cardiolipin, a mitochondrial-specific phospholipid regulating the membrane integrity and activities of the organelle (PubMed:39279645). Has also been shown to display phosphatase activity toward phosphoprotein substrates, specifically mediates dephosphorylation of mitochondrial proteins, thereby playing an essential role in ATP production (By similarity). Has probably a preference for proteins phosphorylated on Ser and/or Thr residues compared to proteins phosphorylated on Tyr residues (By similarity). Probably involved in regulation of insulin secretion in pancreatic beta cells (By similarity). Prevents intrinsic apoptosis, by regulating mitochondrial membrane integrity (PubMed:24709986, PubMed:39279645). {ECO:0000250|UniProtKB:P0C089, ECO:0000250|UniProtKB:Q66GT5, ECO:0000269|PubMed:24709986, ECO:0000269|PubMed:39279645}. This is the function of ENSG00000110536 (PTPMT1, protein tyrosine phosphatase mitochondrial 1).