Microprotein involved in mitochondrial respiratory chain complex III (ubiquinol-cytochrome c oxidoreductase) and complex IV (mitochondrial cytochrome c oxidase complex) assembly (PubMed:35450818). Required for the formation of mitochondrial supercomplexes (SCs) (PubMed:35450818). Also required for the activation of the NLRP3 inflammasome (By similarity). 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 (By similarity). Interacts with components of the cytochrome c oxidase (mitochondrial respiratory chain complex IV) complex, such as MT-CO2 (PubMed:35450818). Expressed in monocytes and dendritic cells. Down-regulated by bacterial lipopolysaccharide (LPS) in monocytes and dendritic cells. Belongs to the STMP1 family.