Microprotein involved in mitochondrial respiratory chain complex III (ubiquinol-cytochrome c oxidoreductase) and complex IV (mitochondrial cytochrome c oxidase complex) assembly (By similarity). Required for the formation of mitochondrial supercomplexes (SCs) (By similarity). Also required for the activation of the NLRP3 inflammasome (By similarity). Widely expressed. Expressed more abundantly in brain compared with other tissues such as heart, muscle and liver. Stable expression throughout embryogenesis with more abundance after 5 days post-fertilization (dpf). Morpholino knockdown of the protein leads to various development defects, including smaller eye, abnormal head shape, malformations of the jaw, cardiac edema and a delayed yolk sac resorption (PubMed:23073385). Most animals die by 1 week of age (PubMed:23073385). Belongs to the STMP1 family.