Molecular characterization of the human COQ5 C-methyltransferase in coenzyme Q10 biosynthesis.

Coq5 catalyzes the only C-methylation involved in the biosynthesis of coenzymeQ(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 withmitochondrial, 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.Weshow that human COQ5 RNA is expressed in all tissues and that the COQ5 polypeptide is associatedwith the mitochondrial innermembrane on thematrix side. Previouswork inyeasthas shownthatpointmutations within or adjacent to conserved COQ5 methyltransferasemotifs result in a loss of Coq5 function but not Coq5 steady state levels. Here, we show that stabilization of the CoQsynthome within coq5 pointmutants or by over-expression of COQ8 in coq5 nullmutants 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 twodimensional blue-native/SDS-PAGE at the same high molecularmass 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.