title: Localization of cytosolically oriented maleimide-reactive domain of vacuolar H(+)-pyrophosphatase.
abstract: The vacuolar H(+)-pyrophosphatase (V-PPase) of plant cells is subject to substrate (Mg2PPi)-protectable, free PPi-potentiated irreversible inhibition by the sulfhydryl reagent N-ethylmaleimide (NEM). Inhibition by NEM approximates pseudo-first order kinetics and double-log plots of the first order rate constant for inactivation versus NEM concentration yield a straight line relationship with a slope of approximately unity. Since NEM and the membrane-impermeant cysteine reagent 3-(N-maleimidylpropionyl)biocytin (MPB) inhibit the V-PPase with similar kinetics and compete for a common binding site on the M(r) = 66,000 substrate-binding subunit, a single residue located in a cytosolically disposed extramembranous domain is inferred to undergo covalent modification in both cases. Selective labeling of the V-PPase of vacuolar membrane vesicles with [14C]NEM, purification of the M(r) = 66,000 subunit, and its digestion with V8 protease generates multiple peptide fragments. Of the bands identified after electrophoresis of the digests on Tris-Tricine gels, only one, migrating at M(r) = 14,000 (V814K), contains 14C label. Gas-phase sequence analysis of this band after electrotransfer to Immobilon PSQ yields two overlapping sequences (V814K2 and V814K2) which unambiguously align with the carboxyl-terminal segment of the M(r) = 66,000 subunit. Both V814K1 and V814K2 encompass only 1 cysteine residue at position 634 which is conserved between the V-PPases from Arabidopsis thaliana, Beta vulgaris (isoforms 1 and 2), and Hordeum vulgare. On the basis of these findings, the strict conservation of the sequence of the V-PPase from multiple plant sources, and the identical kinetics of interaction of the enzymes from Vigna and Beta with NEM and MPB, Cys634 of putative hydrophilic loop X is concluded to be the cytosolically oriented residue whose alkylation by maleimides is responsible for inactivation of the V-PPase. The significance of these results with respect to earlier speculations concerning the identity of the catalytic site and topology of the V-PPase is discussed.
authors.affiliation: Department of Biology, University of Pennsylvania, Philadelphia 19104-6018.
authors.forename: R G
authors.initials: RG
authors.lastname: Zhen
authors.forename: E J
authors.initials: EJ
authors.lastname: Kim
authors.forename: P A
authors.initials: PA
authors.lastname: Rea
mesh_terms: D000595:Amino Acid Sequence; D001667:Binding, Competitive; D002462:Cell Membrane; D002850:Chromatography, Gel; D003600:Cytosol / Q000201:enzymology*; D004591:Electrophoresis, Polyacrylamide Gel; D005033:Ethylmaleimide / Q000494:pharmacology*; D007887:Fabaceae / Q000201:enzymology; D043564:Inorganic Pyrophosphatase; D007700:Kinetics; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000378:metabolism; D010946:Plants, Medicinal; D011487:Protein Conformation; D011755:Pyrophosphatases / Q000037:antagonists & inhibitors / Q000302:isolation & purification / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D012697:Serine Endopeptidases / Q000378:metabolism; D014617:Vacuoles / Q000201:enzymology*
chemical_list: D010446:Peptide Fragments; D012697:Serine Endopeptidases; C028344:glutamyl endopeptidase; D011755:Pyrophosphatases; D043564:Inorganic Pyrophosphatase; D005033:Ethylmaleimide