Human Molecular Genetics Advance Access originally published online on June 30, 2004
Human Molecular Genetics 2004 13(17):1839-1848; doi:10.1093/hmg/ddh197
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Human Molecular Genetics, Vol. 13, No. 17 © Oxford University Press 2004; all rights reserved
Human SCO1 and SCO2 have independent, cooperative functions in copper delivery to cytochrome c oxidase
1Montreal Neurological Institute, 2Department of Human Genetics, McGill University, Montreal, Canada H3A 2B4, 3Department of Pathology and Laboratory Medicine, Children's and Women's Health Centre of British Columbia, Vancouver, Canada V6H 3N1 and 4Metabolic Disease Centre Munich-Schwabing and Institute of Clinical Chemistry, Molecular Diagnostics and Mitochondrial Genetics, 80804 Munich, Germany
Received May 17, 2004; Accepted June 15, 2004
Human SCO1 and SCO2 are paralogous genes that code for metallochaperone proteins with essential, but poorly understood, roles in copper delivery to cytochrome c oxidase (COX). Mutations in these genes produce tissue-specific COX deficiencies associated with distinct clinical phenotypes, although both are ubiquitously expressed. To investigate the molecular function of the SCO proteins, we characterized the mitochondrial copper delivery pathway in SCO1 and SCO2 patient backgrounds. Immunoblot analysis of patient cell lines showed reduced levels of the mutant proteins, resulting in a defect in COX assembly, and the appearance of a common assembly intermediate. Overexpression of the metallochaperone COX17 rescued the COX deficiency in SCO2 patient cells but not in SCO1 patient cells. Overexpression of either wild-type SCO protein in the reciprocal patient background resulted in a dominant-negative phenotype, suggesting a physical interaction between SCO1 and SCO2. Chimeric proteins, constructed from the C-terminal copper-binding and N-terminal matrix domains of the two SCO proteins failed to complement the COX deficiency in either patient background, but mapped the dominant-negative phenotype in the SCO2 background to the N-terminal domain of SCO1, the most divergent part of the two SCO proteins. Our results demonstrate that the human SCO proteins have non-overlapping, cooperative functions in mitochondrial copper delivery. Size exclusion chromatography suggests that both the proteins function as homodimers. We propose a model in which COX17 delivers copper to SCO2, which in turn transfers it directly to the CuA site at an early stage of COX assembly in a reaction that is facilitated by SCO1.
* To whom correspondence should be addressed at: Montreal Neurological Institute, 3801 University Street, Montreal, Quebec, Canada H3A 2B4. Tel: +1 5143988523; Fax: +1 5143981509; Email: eric{at}ericpc.mni.mcgill.ca
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