MUTATIONS IN RCA1 AND AFG3 INHIBIT F1-ATPASE ASSEMBLY IN SACCHAROMYCES-CEREVISIAE

The RCA1 (YTA12) and AFG3 (YTA10) genes of Saccharomyces cerevisiae code for homologous mitochondrial proteins that belong to the recently described AAA protein-family [Kunau et al, (1993) Biochimie 75, 209-224]. Mutations in either gene have been shown to induce a respiratory defect. In the case of rca1 mutants this phenotype has been ascribed to defective assembly of cytochrome oxidase and ubiquinol-cytochrome c reductase. In the present study we show that the respiratory defect of afg3 mutants, like that of rca1 mutants, is also caused by an arrest in assembly of cytochrome oxidase and ubiquinol-cytochrome c reductase. In addition to the absence of the respiratory complexes, rca1 and afg3 mutants exhibit reduced mitochondrial ATPase activity. As a first step to an understanding of the biochemical basis for the ATPase defect we have examined the assembly of the F-1 and F-0 constituents of the ATPase complex. We present evidence that the ATPase lesion stems at least in part from the failure of rca1 and afg3 mutants to assemble F-1. Although the mutants also display lower steady-state concentrations of some F-0 subunits, this could be a secondary effect of defective F-1 assembly.