Acidic residues necessary for pyrophosphate-energized pumping and inhibition of the vacuolar H+-pyrophosphatase by N,N'- dicyclohexylcarbodiimide

Abstract

On the basis of a revised topological model of the vacuolar H+- pyrophosphatase (V-PPase; EC 3.6.1.1) derived from the analysis of four published sequences using two structure-predicting programs, TopPred II and MEMSAT, eight acidic amino acid residues located near or within transmembrane α-helices were identified. The codons specifying these amino acids in the cDNA encoding the V-PPase from Arabidopsis thaliana were singly mutated to examine their involvement in pyrophosphate (PP(i)) hydrolysis and PP(i)- dependent H+ translocation and the functional significance of the similarities between the sequences encompassing Glu229 (227-245) of the V- PPase and the N,N'-dicyclohexylcarbodiimide (DCCD)-binding transmembrane α- helix of the c-subunits of F-ATPases (Nyren, P., Sakai-Nore, Y., and Strid, A. (1993) Plant Cell Physiol. 34, 375-378). Three functional classes were identified after heterologous expression of mutated enzyme in Saccharomyces cerevisiae. Class I (E119Q, E229Q, D573N, E667Q, and E751Q) mutants exhibited PP(i) hydrolytic and H+ translocation activities and DCCD sensitivities similar to wild type. The one class II mutant obtained (E427Q) was preferentially impaired for H+ translocation over PP(i) hydrolysis but retained sensitivity to DCCD. Class HI (E305Q and D504N) mutants exhibited a near complete abolition of both PP(i) hydrolysis and H+ translocation and residual activities with decreased DCCD sensitivity. In none of the mutants was diminished insertion of the V-PPase into the membrane or an increase in the background conductance of the membrane to H+ evident. The decoupled character of E427Q mutants and the enhancement of H+ pumping in E427D mutants by comparison with wild type, in conjunction with the retention of DCCD inhibitability in both E427Q and E427D mutants, implicate a role for Glu427 in DCCD-insensitive H+ translocation by the V-PPase. The proportionate diminution of PP(i) hydrolytic and H+ translocation activity and conservation of Wild type DCCD sensitivity in E229Q mutants refute the notion that Glu229 is the residue whose covalent modification by DCCD is responsible for the abolition of PP(i)-dependent H+ translocation. Instead, the diminished sensitivity of the residual activities of E305Q and D504N mutants, but not E305D or D504E mutants, to inhibition by DCCD is consistent with the involvement of acidic residues at these positions in inhibitory DCCD binding. The results are discussed with regard to the possible involvement of Glu427 in coupling PP(i) hydrolysis with transmembrane H+ translocation and earlier interpretations of the susceptibility of the V-PPase to inhibition by carbodiimides.