Mitochondrial serine protease that acts as a regulator of mitochondrial lipid metabolism (By similarity). Acts by decreasing protein levels of PISD, a mitochondrial enzyme that converts phosphatidylserine (PtdSer) to phosphatidylethanolamine (PtdEtn), thereby affecting mitochondrial lipid metabolism (By similarity). It is unclear whether it acts directly by mediating proteolysis of PISD or by mediating proteolysis of another lipid metabolism protein (By similarity). Acts as a tumor suppressor that has the ability to inhibit proliferation of multiple types of cancer cells: probably by promoting decreased levels of PISD, thereby affecting mitochondrial lipid metabolism (PubMed:28329758). Expressed predominantly in liver. Down-regulated in a number of cancer cells. Belongs to the peptidase S12 family.