Subunit f of the yeast mitochondrial ATP synthase: Topological and functional studies

Modified versions of subunit f were produced by mutagenesis of the ATP17 ge ne of Saccharomyces cerevisiae. A version of subunit f devoid of the last 2 8 amino acid residues including the unique transmembranous domain complemen ted the oxidative phosphorylation of the null mutant. However, a two-fold d ecrease in the specific ATP synthase activity was measured and attributed t o a decrease in the stability of the mutant ATP synthase complex as shown b y the low oligomycin-sensitive ATPase activity at alkaline pH. The modifica tion or not by nonpermeant maleimide reagents of cysteine residues introduc ed at the N and C termini of subunit f indicated a N-in-C-out orientation. From the C terminus of subunit f it was possible to crosslink subunit 4 (al so called subunit b), which is another component of the F-o sector and whic h also displays a short hydrophilic segment exposed to the intermembrane sp ace.