Cysteine Scanning Mutagenesis of the Noncatalytic Nucleotide Binding Site of the Yeast V-ATPase

Abstract

To investigate residues involved in the formation of the noncatalytic nucleotide binding sites of the vacuolar proton-translocating adenosine triphosphatase (V-ATPase), cysteine scanning mutagenesis of the VMA2 gene that encodes the B subunit in yeast was performed. Replacement of the single endogenous cysteine residue at position 188 gave rise to a Cys-less form of the B subunit (Vma2p) which had near wild-type levels of activity and which was used in the construction of 16 single cysteine-containing mutants. The ability of adenine nucleotides to prevent reaction of the introduced cysteine residues with the sulfhydryl reagent 3-(N-maleimidopropionyl)biocytin (biotin-maleimide) was evaluated by Western blot. Biotin-maleimide labeling of the purified V-ATPase from the wild-type and the mutants S152C, L178C, N181C, A184C, and T279C was reduced after reaction with the nucleotide analog 3'-O-(4-benzoyl)benzoyladenosine 5'-triphosphate (BzATP). These results suggest the proximity of these residues to the nucleotide binding site on the B subunit. In addition, we have examined the level of endogenous nucleotide bound to the wild-type V-ATPase and to a mutant (the A subunit mutant R483Q) which is postulated to be altered at the noncatalytic site and which displays a marked nonlinearity in ATP hydrolysis (MacLeod, K. J., Vasilyeva, E., Baleja, J. D., and Forgac, M. (1998) J. Biol. Chem. 273, 150-156). The R483Q mutant contained 2.6 mol of ATP/mol of V-ATPase compared with the wild-type enzyme, which contained 0.8 mol of ATP/mol of V-ATPase. These results suggest that binding of additional ATP to the noncatalytic sites may modulate the catalytic activity of the enzyme.