The mitochondrial protein TMEM177 fine-tunes mammalian cytochrome c oxidase assembly

• Depletion of mitochondrial protein TMEM177 in mice results in reduced COX20 levels. • Loss of TMEM177 results in a proportion of mice dying perinatally. • Surviving TMEM177 knockout mice tend to gain less weight. • OXPHOS assembly and activity are preserved upon loss of TMEM177. • TMEM177 interacts with subunits of complex I, complex IV, and the mitoribosome. The mitochondrial cytochrome c oxidase (COX, complex IV), a multi-subunit protein complex, plays a crucial role in cellular respiration by reducing oxygen to water and simultaneously pumping protons to enable oxidative phosphorylation (OXPHOS). Thus, defects in its assembly can directly affect cellular energy homeostasis. COX20 is an essential chaperone for the core subunit COX2. In human cultured cells, TMEM177 was found to stabilize COX20 and maintain balanced COX2 levels. In mice, TMEM177 was also identified as an interactor of mitochondrial ribosomes. To understand the function of TMEM177 in vivo , we generated Tmem177 knockout mice. Here, we analyze how TMEM177 loss affects mitochondrial gene expression, as well as the activity and assembly of OXPHOS complexes. We found that a small proportion of the knockout mice died perinatally, while surviving knockout mice tended to gain less weight. TMEM177 depletion moderately reduced COX20 levels, but OXPHOS complexes were preserved. Moreover, Tmem177 and Surf1 double knockout mice were born asymptomatic. In conclusion, TMEM177 fine-tunes complex IV assembly by stabilizing COX20 in vivo . Our findings refine the current model of complex IV assembly in mammals.