Identification of additional homologues of subunits VII and VIII of the ubiquinol‐cytochrome c oxidoreductase enables definition of consensus sequences

Abstract

The Candida utilis QCR7 gene encoding subunit VII of the ubiquinol-cytochrome c oxidoreductase was isolated by functional complementation of the Saccharomyces cerevisiae subunit VII-null mutant. Several other subunit VII homologues as well as homologues for subunit VIII were identified by screening the GenBank database. Some of these homologues for subunit VII could only be identified as such using a consensus sequence that was derived from the multiple sequence alignment. Definition of the consensus should facilitate further analysis of structure/ function relationships in this protein.

Subunit VII of ubiquinol:cytochrome-c oxidoreductase from Neurospora crassa is functional in yeast and has an N-terminal extension that is not essential for mitochondrial targeting

“…In comparison, the QCR8 product of Kluy eromyces lactis complements as well as S. cere isiae QCR8p [32], whereas the products of the bovine and potato QCR8 homologues do not complement at all (H. Boumans and G. Lobo-Hajdu, unpublished work). These findings suggest that neither the consensus sequence [9] nor the conserved structural features contains all the elements necessary for proper assembly. Assuming that the consensus sequence is required for the functionality of an assembled complex, we can postulate that, in addition to the overall structure, the sequence of the regions of interaction with the other subunits of the complex determines the level of assembly.…”

“…Gaps are indicated by dots. Sequence origins : 1, Solanum tuberosum [34] ; 2, Oryza sativa ( [9], and GenBank accession number D48989) ; 3, Arabidopsis thaliana [9] ; 4, bovine heart [33] ; 5, human (GenBank accession number T47406) ; 6, S. cerevisiae [6] ; 7, K. lactis [32] ; 8, Schizosaccharomyces pombe [20] ; and 9, N. crassa (this study). 48 %.…”

“…48 %. The addition of the N. crassa and human sequences to the list of homologues results in some differences in the alignment compared with [9]. However, the most striking feature of this alignment is the presence in N. crassa of a short N-terminal extension that is absent from all its counterparts.…”

“…Computer screening of databases has led to the identification of homologues of QCR8p in a wide variety of organisms. The protein family displays a low degree of sequence conservation, but sequence alignments have allowed the identification of a number of conserved residues and the formulation of a characteristic signature [9]. To assess the functional equivalence of the different evolutionary variants we have followed an approach involving the construction of site-directed mutants, based on the consensus sequence, the construction of hybrid complexes containing QCR8p homologues in a S. cere isiae background and a study of their enzymic properties.…”

Functional complementation analysis of yeastbc1mutants

“…Subunit‐VII orthologues from organisms much more closely related to S. cerevisiae , such as Kluyeromyces lactis [43] and Candida utilis [44], are able to complement a qcr7 null mutant, indicating a correlation between evolutionary distance from S. cerevisiae and the ability of functional complementation. As this distance is much larger between S. cerevisiae and man, the evolutionary divergence of the import apparatus of these organisms may account for the lack of complementation of the qcr7 null mutant and for the lack of in vitro import into S. cerevisiae mitochondria of the human orthologue of subunit VII.…”