Microprotein involved in mitochondrial respiratory chain complex III (ubiquinol-cytochrome c oxidoreductase) and complex IV (mitochondrial cytochrome c oxidase complex) assembly (PubMed:35101990). Required for the formation of mitochondrial supercomplexes (SCs) (PubMed:35101990). Also required for the activation of the NLRP3 inflammasome (PubMed:31836654). Interacts with components of the ubiquinol-cytochrome c oxidoreductase (cytochrome b-c1 complex, complex III, CIII), such as UQCRC1/QCR1, UQCRC2/QCR2 and UQCR10/QCR9 (PubMed:35101990). Interacts with components of the cytochrome c oxidase (mitochondrial respiratory chain complex IV) complex, such as MT-CO2 (By similarity). Widely expressed, with highest levels in lung, skeletal muscle and heart (PubMed:31836654, PubMed:35101990). Expressed in bone marrow-derived macrophages (at protein level) (PubMed:31836654). Down-regulated by TLR ligands, including bacterial lipopolysaccharide (LPS), in bone marrow-derived macrophages. Mice were born at expected Mendelian ratios and do not show any visible phenotype, displaying normal heart function and cardiac dimensions (PubMed:35101990). Cells however display defects in complex III activity and metabolic perturbations in the heart, as well as altered complex III assembly into respiratory supercomplexes (PubMed:35101990). Belongs to the STMP1 family. Wrong choice of CDS.