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 ␣-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 The membranes constituting the vacuolysosomal complex of plant cells are unusual in possessing an H ϩ translocating inorganic pyrophosphatase (V-PPase 1 ; EC 3.6.1.1) (2). The VPPase bears no systematic resemblance to soluble PPases at the sequence level (3, 4) and is considered to belong to a fourth class of H ϩ -phosphohydrolase distinct from the F-, P-and VATPases (4). Moreover, unlike the V-ATPase, which is ubiquitous in the membranes bounding the acidic intracellular compartments of all eukaryotic cells, the V-PPase appears to be restricted to plants and a few species of phototrophic bacteria (2, 5). Notwithstanding the intrinsic evolutionary interest of this phenomenon, it poses a problem: the lack of sequencedivergent homologs from phylogenically remote organisms. Because all published V-PPase sequences are from the same group of organisms, vascular plants, and exhibit greater than 85% sequence identity at the amino acid level (6), most attempts to identify conserved amino acid residues of potential mechanistic significance by sequence alignment procedures have been unproductive. Crucial, therefore, has been the development of methods for the expression of functional pump in the yeast, Saccharomyces cerevisiae (7,8). When constructs of the yeast-Escherichia coli shuttle vector pYES2, containing the entire open reading frame of the cDNA (AVP; Ref. 9) encoding the M r 66,000 substrate-binding subunit 2 of the V-PPase from Arabidopsis thaliana are employed to transform S. cerevisiae, endomembrane-associated enzyme active in PP i -dependent H ϩ translocation is generated (7). Since the heterologously expressed pump is indistinguishable from the native plant enzyme, thereby establishing the sufficiency of AVP for the elab-* This work was supported by Grant MCB93-05281 from the National Science Foundation (NSF) and Grant DE-FG02-91ER20055 from the Department of Energy (DOE) (both to P. A. R.) and DOE/NSF/USDA Triagency Plant Training Grant DE-FG02-94ER20162 awarded to the Plant Science Institute. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.)‡ To whom correspondence should be addressed. E-mail: parea@sas.upenn.edu. 1 The abbreviations used are: V-PPase, vacuolar H ϩ -pyrophosphatase; DCCD, N,NЈ-dicyclohexylcarbodiimide; Mes, 4-morpholineethanesulfonic acid; NEM, N-ethylmaleimide; EDAC, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide; V-, F-, and P-ATPases,...
