title: Molecular characterization of the human COQ5 C-methyltransferase in coenzyme Q10 biosynthesis.
abstract: Coq5 catalyzes the only C-methylation involved in the biosynthesis of coenzyme Q (Q or ubiquinone) in humans and yeast Saccharomyces cerevisiae. As one of eleven polypeptides required for Q production in yeast, Coq5 has also been shown to assemble with the multi-subunit complex termed the CoQ-synthome. In humans, mutations in several COQ genes cause primary Q deficiency, and a decrease in Q biosynthesis is associated with mitochondrial, cardiovascular, kidney and neurodegenerative diseases. In this study, we characterize the human COQ5 polypeptide and examine its complementation of yeast coq5 point and null mutants. We show that human COQ5 RNA is expressed in all tissues and that the COQ5 polypeptide is associated with the mitochondrial inner membrane on the matrix side. Previous work in yeast has shown that point mutations within or adjacent to conserved COQ5 methyltransferase motifs result in a loss of Coq5 function but not Coq5 steady state levels. Here, we show that stabilization of the CoQ-synthome within coq5 point mutants or by over-expression of COQ8 in coq5 null mutants permits the human COQ5 homolog to partially restore coq5 mutant growth on respiratory media and Q6 content. Immunoblotting against the human COQ5 polypeptide in isolated yeast mitochondria shows that the human Coq5 polypeptide migrates in two-dimensional blue-native/SDS-PAGE at the same high molecular mass as other yeast Coq proteins. The results presented suggest that human and Escherichia coli Coq5 homologs expressed in yeast retain C-methyltransferase activity but are capable of rescuing the coq5 yeast mutants only when the CoQ-synthome is assembled.
authors.affiliation: Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095-1569, USA; Molecular Biology Institute, University of California, Los Angeles, CA 90095-1569, USA.
authors.forename: Theresa P T
authors.initials: TP
authors.lastname: Nguyen
authors.affiliation: Clinical Genetics Unit, Dept. of Pediatrics, University of Padova, Via Giustiniani 3, Padova 35128, Italy.
authors.forename: Alberto
authors.initials: A
authors.lastname: Casarin
authors.affiliation: Clinical Genetics Unit, Dept. of Pediatrics, University of Padova, Via Giustiniani 3, Padova 35128, Italy.
authors.forename: Maria Andrea
authors.initials: MA
authors.lastname: Desbats
authors.affiliation: Clinical Genetics Unit, Dept. of Pediatrics, University of Padova, Via Giustiniani 3, Padova 35128, Italy.
authors.forename: Mara
authors.initials: M
authors.lastname: Doimo
authors.affiliation: Clinical Genetics Unit, Dept. of Pediatrics, University of Padova, Via Giustiniani 3, Padova 35128, Italy.
authors.forename: Eva
authors.initials: E
authors.lastname: Trevisson
authors.affiliation: Centro Andaluz de Biología del Desarrollo, Universidad Pablo de Olavide-CSIC, CIBERER, Instituto de Salud Carlos III, Sevilla, Spain.
authors.forename: Carlos
authors.initials: C
authors.lastname: Santos-Ocaña
authors.affiliation: Centro Andaluz de Biología del Desarrollo, Universidad Pablo de Olavide-CSIC, CIBERER, Instituto de Salud Carlos III, Sevilla, Spain.
authors.forename: Placido
authors.initials: P
authors.lastname: Navas
authors.affiliation: Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095-1569, USA; Molecular Biology Institute, University of California, Los Angeles, CA 90095-1569, USA. Electronic address: cathy@chem.ucla.edu.
authors.forename: Catherine F
authors.initials: CF
authors.lastname: Clarke
authors.affiliation: Clinical Genetics Unit, Dept. of Pediatrics, University of Padova, Via Giustiniani 3, Padova 35128, Italy. Electronic address: leonardo.salviati@unipd.it.
authors.forename: Leonardo
authors.initials: L
authors.lastname: Salviati
keywords: Human COQ gene; Mitochondrial metabolism; Protein complex; Q-biosynthetic intermediate; Saccharomyces cerevisiae; Ubiquinone