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A CMC1‐knockout reveals translation‐independent control of human mitochondrial complex IV biogenesis

Defects in mitochondrial respiratory chain complex IV (CIV) frequently cause encephalocardiomyopathies. Human CIV assembly involves 14 subunits of dual genetic origin and multiple nucleus‐encoded ancillary factors. Biogenesis of the mitochondrion‐encoded copper/heme‐containing COX1 subunit initiates the CIV assembly process. Here, we show that the intermembrane space twin CX9C protein CMC1 forms an early CIV assembly intermediate with COX1 and two assembly factors, the cardiomyopathy proteins COA3 and COX14. A TALEN‐mediated CMC1 knockout HEK293T cell line displayed normal COX1 synthesis but decreased CIV activity owing to the instability of newly synthetized COX1. We demonstrate that CMC1 stabilizes a COX1‐COA3‐COX14 complex before the incorporation of COX4 and COX5a subunits. Additionally, we show that CMC1 acts independently of CIV assembly factors relevant to COX1 metallation (COX10, COX11, and SURF1) or late stability (MITRAC7). Furthermore, whereas human COX14 and COA3 have been proposed to affect COX1 mRNA translation, our data indicate that CMC1 regulates turnover of newly synthesized COX1 prior to and during COX1 maturation, without affecting the rate of COX1 synthesis.