Sco1 and Sco2 are mitochondrial
copper-
binding proteins involved in the biogenesis of the Cu(A) site in the
cytochrome c oxidase (CcO) subunit Cox2 and in the maintenance of cellular
copper homeostasis. Human Surf1 is a CcO assembly factor with an important but poorly characterized role in CcO biogenesis. Here, we analyzed the impact on CcO assembly and tissue
copper levels of a G132S mutation in the juxtamembrane region of SCO1
metallochaperone associated with early onset
hypertrophic cardiomyopathy,
encephalopathy,
hypotonia, and hepatopathy, assessed the total
copper content of various SURF1 and SCO2-deficient tissues, and investigated the possible physical association between CcO and Sco1. The steady-state level of mutant Sco1 was severely decreased in the muscle mitochondria of the SCO1 patient, indicating compromised stability and thus loss of function of the
protein. Unlike the wild-type variant, residual mutant Sco1 appeared to migrate exclusively in the monomeric form on blue native
gels. Both the activity and content of CcO were reduced in the patient's muscle to approximately 10-20% of control values. SCO1-deficient mitochondria showed accumulation of two Cox2 subcomplexes, suggesting that Sco1 is very likely responsible for a different posttranslational aspect of Cox2 maturation than Sco2. Intriguingly, the various SURF1-deficient samples analyzed showed a tissue-specific
copper deficiency similar to that of SCO-deficient samples, suggesting a role for Surf1 in
copper homeostasis regulation. Finally, both blue native immunoblot analysis and coimmunoprecipitation revealed that a fraction of Sco1 physically associates with the CcO complex in human muscle mitochondria, suggesting a possible direct relationship between CcO and the regulation of cellular
copper homeostasis.