PPM1K exhibited phosphatase activity against a synthetic substrate in the presence of Mn(2+), but not Mg(2+). It showed phosphoserine phosphatase activity against the branched-chain alpha-keto acid dehydrogenase complex, suggesting it may play a role in regulating branched-chain amino acid catabolism.The deduced 372-amino acid protein contains an N-terminal mitochondrial localization signal and a central PP2C catalytic domain. Northern blot analysis of mouse tissues detected highest expression of Pp2cm in adult heart, brain, and diaphragm, with lower levels in liver, lung, kidney, skeletal muscle, and thymus. Immunohistochemical analysis of neonatal rat cardiomyocytes revealed colocalization of Pp2cm with a mitochondrial marker.