biostudies-literatureBourens MAmerican Heart AssociationNational Institute of General Medical SciencesNIGMS NIH HHSMuscular Dystrophy Association477-494https://www.ebi.ac.uk/biostudies/studies/S-EPMC5331208biostudies-literatureUnknown18(3)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 <i>CMC1</i> 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 subEMBO reportsA <i>CMC1</i>-knockout reveals translation-independent control of human mitochondrial complex IV biogenesis.PMC5331208R01 GM071775MDA‐381828R01 GM112179GM071775R35GM118141GM112179R01 GM105781GM105781R35 GM118141Bourens MBarrientos AfalseA <i>CMC1</i>-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 <i>CMC1</i> 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 sub2017-01-01T00:00:00Z2017 Mar2026-05-05T19:40:30.169Z2019-03-26T23:04:10ZS-EPMC53312082808231410.15252/embr.201643103