Mitochondrial serine protease that acts as a regulator of mitochondrial lipid metabolism (PubMed:28329758). Acts by decreasing protein levels of PISD, a mitochondrial enzyme that converts phosphatidylserine (PtdSer) to phosphatidylethanolamine (PtdEtn), thereby affecting mitochondrial lipid metabolism (PubMed:28329758). It is unclear whether it acts directly by mediating proteolysis of PISD or by mediating proteolysis of another lipid metabolism protein (PubMed:28329758). Acts as a tumor suppressor that has the ability to inhibit proliferation of multiple types of breast cancer cells: probably by promoting decreased levels of PISD, thereby affecting mitochondrial lipid metabolism (PubMed:28329758). Expressed predominantly in skeletal muscle. Down-regulated in a number of cancer cells. Belongs to the peptidase S12 family. Was originally identified as mitochondrial large ribosomal subunit protein mL56 (MRP-L56) (PubMed:11551941), but has since been shown to localize to the mitochondrial intermembrane space where it forms filaments (PubMed:19858488).