Genetic evidence for a multi-subunit complex in the O-methyltransferase steps of coenzyme Q biosynthesis

Coq3 O-methyltransferase carries out both O-methylation steps in coenzyme Q (ubiquinone) biosynthesis. The degree to which Coq3 O-methyltransferase ac tivity and expression are dependent on the other seven COQ gene products ha s been investigated. A panel of yeast mutant strains harboring null mutatio ns in each of the genes required for coenzyme Q biosynthesis (COQ1-COQ8) ha ve been prepared. Mitochondria have been isolated from each member of the y east cog mutant collection, from the wild-type parental strains and from re spiratory deficient mutants harboring deletions in ATP2 or COR1 genes. Thes e latter strains constitute Q-replete, respiratory deficient controls. Each of these mitochondrial preparations has been analyzed for COQ3-encoded O-m ethyltransferase activity and steady state levels of Coq3 polypeptide. The findings indicate that the presence of the other COQ gene products is requi red to observe normal levels of O-methyltransferase activity and the Coq3 p olypeptide. However, COQ3 steady state RNA levels are not decreased in any of the cog mutants, relative to either wild-type or respiratory deficient c ontrol strains, suggesting either a decreased rate of translation or a decr eased stability of the Coq3 polypeptide. These data are consistent with the involvement of the Coq polypeptides (or the Q-intermediates formed by the Coq polypeptides) in a multi-subunit complex. It is our hypothesis that a d eficiency in any one of the COQ gene products results in a defective comple x in which the Coq3 polypeptide is rendered unstable. (C) 2000 Elsevier Sci ence B.V. All rights reserved.