Characterization of mutations in the beta subunit of the mitochondrial F1-ATPase that produce defects in enzyme catalysis and assembly.

The ATP2 gene, coding for the beta subunit of the mitochondrial F1-ATPase, was cloned from nine independent isolates of chemically mutagenized yeast. Seven different mutant alleles were identified. In one case the mutation occurs in the mitochondrial targeting sequence (M1I). The remaining six mutations map to the mature part of the beta subunit protein and alter amino acids that are conserved in the bovine heart mitochondrial and Escherichia coli beta subunit proteins. Biochemical analysis of the yeast atp2 mutants identified two different phenotypes. The G133D, P179L, and G227D mutations correlate with an assembly-defective phenotype that is characterized by the accumulation of the F1 alpha and beta subunits in large protein aggregates. Strains harboring the A192V, E222K, or R293K mutations assemble an F1 of normal size that is none-the-less catalytically inactive. The effect of the atp2 mutations was also analyzed in diploids formed by crossing the mutants to wild type yeast. Hybrid enzymes formed with beta subunits containing either the G133D, E222K, or R293K mutations are compromised for steady-state ATPase activity. The display of partial dominance confirms the importance of Gly133 for structural stability and of Glu222 and Arg293 for catalytic cooperativity.