Roles of Oxa1-related inner-membrane translocases in assembly of respiratory chain complexes

Abstract

Members of the family of the polytopic inner membrane proteins are related to Saccharomyces cerevisiae Oxa1 function in the assembly of energy transducing complexes of mitochondria and chloroplasts. Here we focus on the two mitochondrial members of this family, Oxa1 and Cox18, reviewing studies on their biogenesis as well as their functions, reflected in the phenotypic consequences of their absence in various organisms. In yeast, cytochrome c oxidase subunit II (Cox2) is a key substrate of these proteins. Oxa1 is required for co-translational translocation and insertion of Cox2, while Cox18 is necessary for the export of its C-terminal domain. Genetic and biochemical strategies have been used to investigate the functions of distinct domains of Oxa1 and to identify its partners in protein insertion/translocation. Recent work on the related bacterial protein YidC strongly indicates that it is capable of functioning alone as a translocase for hydrophilic domains and an insertase for TM domains. Thus, the Oxa1 and Cox18 probably catalyze these reactions directly in a co- and/or posttranslational way. In various species, Oxa1 appears to assist in the assembly of different substrate proteins, although it is still unclear how Oxa1 recognizes its substrates, and whether additional factors participate in this beyond its direct interaction with mitochondrial ribosomes, demonstrated in S. cerevisiae. Oxa1 is capable of assisting posttranslational insertion and translocation in isolated mitochondria, and Cox18 may posttranslationally translocate its only known substrate, the Cox2 C-terminal domain, in vivo. Detailed understanding of the mechanisms of action of these two proteins must await the resolution of their structure in the membrane and the development of a true in vitro mitochondrial translation system.

Figure 1

Comparison of the general architectures of Oxa1-related proteins. The relative spacing of helices number 2–4 is largely conserved. A large internal C-terminal domain is present in mitochondrial Oxa1 and chloroplast Alb3, but absent in mitochondrial Cox18 and bacterial YidC. E. coli YidC has an N-terminal ‘anchor’ TM domain and long periplasmic N-terminal loop not found in the organellar proteins or related proteins from Gram positive bacteria. See text for references.

Figure 2

Evolutionary relationships among eukaryotic Oxa1-related proteins. This figure is adapted from the analysis of [17] to illustrate the apparent early divergence of Oxa1 and Cox18 subfamilies in the eukaryotic lineage. It is important to note that similarities among closely related proteins within subfamilies are more complex than shown here: the Oxa1 and Cox18 proteins of Schizosaccharomyces pombe are more closely similar to those of most animals than other fungi, while the Oxa1 and Cox18 proteins of Caenorhabditis elegans are more closely similar to those of most fungi than other animals [17]. Chloroplast Alb3 and the mitochondrial proteins presumably entered eukaryotic lineages separately from different bacterial ancestors.

Figure 3

Mutations affecting S. cerevisiae Oxa1. Phenotypes caused by these mutations are summarized in table 1.

Figure 4

Summary of S. cerevisiae mitochondrial proteins that play roles in topogenesis of Cox2 in association with Oxa1 and Cox18. Overproduction of Rmd9 or Oms1 suppresses partially functional alleles of Oxa1. Mba1 may assist in establishing productive interaction between Oxa1 and nascent Cox2, and/or the ribosome. Cleavage of the pre-Cox2 leader peptide by Imp1 in the IMS is required for full assembly of Cox2 and provides an assay for N-tail export by Oxa1. Mss2 and Pnt1 interact with Cox18. Like Cox18, Mss2 is required for Cox2 C-tail export and interacts with newly synthesized Cox2. Purple dots in the fully translocated form of Cox2 indicate copper ions in the C-tail. See text for further discussion and references.