No abstract

Deletion of the Saccharomyces cerevisiae gene YOL008W, here referred to as COQ10, elicits a respiratory defect as a result of the inability of the mutant to oxidize NADH and succinate. Both activities are restored by exogenous coenzyme Q 2 . Respiration is also partially rescued by COQ2, COQ7, or COQ8/ABC1, when these genes are present in high copy. Unlike other coq mutants, all of which lack Q 6 , the coq10 mutant has near normal amounts of Q 6 in mitochondria. Coq10p is widely distributed in bacteria and eukaryotes and is homologous to proteins of the "aromatic-rich protein family" Pfam03654 and to members of the START domain superfamily that have a hydrophobic tunnel implicated in binding lipophilic molecules such as cholesterol and polyketides. Analysis of coenzyme Q in polyhistidine-tagged Coq10p purified from mitochondria indicates the presence 0.032-0.034 mol of Q 6 /mol of protein. We propose that Coq10p is a Q 6 -binding protein and that in the coq10 mutant Q 6 it is not able to act as an electron carrier, possibly because of improper localization.Coenzyme Q (ubiquinone) is an essential electron carrier of the mitochondrial respiratory chain. Its main function is to accept electrons from the NADH-and succinate-coenzyme Q reductases and to donate them to the bc 1 complex (1). Biosynthesis of coenzyme Q in eukaryotes occurs in mitochondria. The benzoquinone ring of coenzyme Q 6 (Q 6 ) of Saccharomyces cerevisiae has a polyprenyl side chain with six isoprenoid units (2). At least nine yeast nuclear genes (COQ1-9) defined by nine complementation groups of a pet 3 mutant collection have been inferred to participate in the synthesis of Q 6 based on the biochemical properties of the mutant mitochondria (3, 4). Mutations in each of the nine genes block the NADH and succinate-cytochrome c reductase activities of mitochondria, which can be restored by addition of coenzyme Q 2 (5). Additionally, coq mutants lack Q 6 and coq3, coq4, coq5, coq6, coq7, coq8/abc1, and coq9 mutants accumulate 3-hexaprenyl-4-hydroxybenzoic acid, an early intermediate in Q 6 biosynthesis (Refs. 4 and 6 -13, see Ref. 11 for details of the pathway in yeast). Except for demethoxy-Q 6 , which accumulates in certain coq7 point mutants (7,14), other intermediates of the pathway are not detected in coq mutants (6 -12). These observations indicate that the pathway is stringently regulated and/or that most of the intermediates are degraded when biosynthesis of Q 6 is arrested. COQ gene products are located in, or are peripherally associated with the inner membrane of mitochondria (9 -15) where they constitute a pathway that is similar to but diverges from the one in bacteria in at least two steps (16,17).In the present study we report a novel phenotype displayed by mutants with a deletion of reading frame YOL008W, which we have named COQ10. The coq10 mutant is similar to other coq mutants in which optimal oxidation of NADH and succinate by isolated mitochondria depends on addition of coenzyme Q 2 . Unlike the coq mutants, which lack Q 6 , the coq...

Coenzyme Q (Q) is a redox active lipid that is an essential component of the electron transport chain. Here, we show that steady state levels of Coq3, Coq4, Coq6, Coq7 and Coq9 polypeptides in yeast mitochondria are dependent on the expression of each of the other COQ genes. Submitochondrial localization studies indicate Coq9p is a peripheral membrane protein on the matrix side of the mitochondrial inner membrane. To investigate whether Coq9p is a component of a complex of Qbiosynthetic proteins, the native molecular mass of Coq9p was determined by Blue Native-PAGE. Coq9p was found to co-migrate with Coq3p and Coq4p at a molecular mass of approximately 1 MDa. A direct physical interaction was shown by the immunoprecipitation of HA-tagged Coq9 polypeptide with Coq4p, Coq5p, Coq6p and Coq7p. These findings, together with other work identifying Coq3p and Coq4p interactions, identify at least six Coq polypeptides in a multi-subunit Q biosynthetic complex.

Ubiquinone (coenzyme Q) is a lipid that transports electrons in the respiratory chains of both prokaryotes and eukaryotes. Mutants of Saccharomyces cerevisiae deficient in ubiquinone biosynthesis fail to grow on nonfermentable carbon sources and have been classified into eight complementation groups (coq1 coq8; Tzagoloff, A., and Dieckmann, C. L.(1990) Microbiol. Rev. 54, 211-225). In this study we show that although yeast coq7 mutants lack detectable ubiquinone, the coq7 1 mutant does synthesize demethoxyubiquinone (2-hexaprenyl-3-methyl-6-methoxy-1,4-benzoquinone), a ubiquinone biosynthetic intermediate. The corresponding wild-type COQ7 gene was isolated, sequenced, and found to restore growth on nonfermentable carbon sources and the synthesis of ubiquinone. The sequence predicts a polypeptide of 272 amino acids which is 40% identical to a previously reported Caenorhabditis elegans open reading frame. Deletion of the chromosomal COQ7 gene generates respiration defective yeast mutants deficient in ubiquinone. Analysis of several coq7 deletion strains indicates that, unlike the coq7 1 mutant, demethoxyubiquinone is not produced. Both coq7 1 and coq7 deletion mutants, like other coq mutants, accumulate an early intermediate in the ubiquinone biosynthetic pathway, 3-hexaprenyl-4-hydroxybenzoate. The data suggest that the yeast COQ7 gene may encode a protein involved in one or more monoxygenase or hydroxylase steps of ubiquinone biosynthesis.