Essential for the assembly of the mitochondrial respiratory chain complex IV (CIV), also known as cytochrome c oxidase. Acts as a chaperone in the early steps of cytochrome c oxidase subunit II (MT-CO2/COX2) maturation, stabilizing the newly synthesized protein and presenting it to metallochaperones SCO1/2 which in turn facilitates the incorporation of the mature MT-CO2/COX2 into the assembling CIV holoenzyme. Found in a complex with TMEM177, COA6, MT-CO2/COX2, COX18, SCO1 and SCO2. Interacts with SCO1, SCO2 and COA6 in a MT-CO2/COX2- and COX18-dependent manner. Interacts with COX18 in a MT-CO2/COX2-dependent manner. Interacts with MT-CO2/COX2 and TMEM177. Belongs to the COX20 family.