Site-directed mutagenesis of vacuolar H(+)-pyrophosphatase. Necessity of Cys634 for inhibition by maleimides but not catalysis

Identification of cysteine and arginine residues essential for the phosphotransacetylase from Methanosarcina thermophila

Phosphotransacetylase catalyzes the following reaction: CoASH + CH3CO2PO3(2-) <==> CH3COSCoA + HPO4(2-) (where CoA is coenzyme A). Based on biochemical characterization of the enzyme from the obligate anaerobe Clostridium kluyveri, a ternary mechanism was proposed in which an unspecified cysteine abstracts a proton from CoASH forming a nucleophilic thiolate anion which attacks acetyl phosphate (J. Henkin and R. H. Abeles, Biochemistry 15:3472-3479, 1976). Heterologous production in Escherichia coli of the phosphotransacetylase from Methanosarcina thermophila, an obligately anaerobic methanoarchaeon, allowed site-specific replacements to identify essential residues. All four cysteines present in the sequence were individually replaced with alanine, and the kinetic constants of the altered enzymes were determined. The results indicated that only C159 is essential for activity; however, replacement with serine resulted in a fully active enzyme. Activity of the unaltered phosphotransacetylase was sensitive to N-ethylmaleimide. Inhibition kinetics of altered enzymes indicated that this sensitivity resulted from modification of C312, which is at the active site but itself is nonessential for catalysis. Five arginines were individually replaced with glutamine. Kinetic analysis of the altered enzymes identified R310 as essential for activity. Of the four nonessential for activity, R87 and R133 appear to be involved in binding CoA.

Purification and characterization of thylakoid membrane-bound inorganic pyrophosphatase from Spinacia oleracia L

An inorganic pyrophosphatase (PPase) was purified from thylakoid membrane of spinach leaves to electrophoretic purity by methods including detergent solubilization, ammonium sulfate fractionation, and successive chromatographic techniques. Current protocol yielded about 10% recovery of total activity with a 30-fold purification. The specific activity of the purified enzyme was approximately 400 micromol PPi consumed/mg protein x h. This enzyme is a monomer with a molecular mass of 55 kDa. Several properties, including subunit composition, substrate specificity, ion requirements, inhibitor sensitivities, and amino acid composition, have been studied. Mg2+ is an essential cofactor for the thylakoid PPase. The preferred substrate for the hydrolytic reaction of PPase appears to be dimagnesium pyrophosphate. K+ could not stimulate the enzymatic activity of thylakoid PPase, while F- was a potent inhibitor. Group-specific modification of the thylakoid PPase demonstrates possible involvement of carboxylate residues in the enzymatic activity. Furthermore, antibodies raised against thylakoid PPase in a rabbit could inactivate the PPi hydrolysis of thylakoid and the purified enzyme, but not that of vacuolar H+-PPase, indicating both PPi hydrolases are structurally distinct.

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

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 alpha-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 (PPi) hydrolysis and PPi-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 alpha-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 PPi 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 PPi hydrolysis but retained sensitivity to DCCD. Class III (E305Q and D504N) mutants exhibited a near complete abolition of both PPi 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 PPi 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 PPi-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 PPi hydrolysis with transmembrane H+ translocation and earlier interpretations of the susceptibility of the V-PPase to inhibition by carbodiimides.

AtMRP1 gene of Arabidopsis encodes a glutathione S-conjugate pump: isolation and functional definition of a plant ATP-binding cassette transporter gene

Because plants produce cytotoxic compounds to which they, themselves, are susceptible and are exposed to exogenous toxins (microbial products, allelochemicals, and agrochemicals), cell survival is contingent on mechanisms for detoxifying these agents. One detoxification mechanism is the glutathione S-transferase-catalyzed glutathionation of the toxin, or an activated derivative, and transport of the conjugate out of the cytosol. We show here that a transporter responsible for the removal of glutathione S-conjugates from the cytosol, a specific Mg2+-ATPase, is encoded by the AtMRP1 gene of Arabidopsis thaliana. The sequence of AtMRP1 and the transport capabilities of membranes prepared from yeast cells transformed with plasmid-borne AtMRP1 demonstrate that this gene encodes an ATP-binding cassette transporter competent in the transport of glutathione S-conjugates of xenobiotics and endogenous substances, including herbicides and anthocyanins.

A new pathway for vacuolar cadmium sequestration in Saccharomyces cerevisiae: YCF1-catalyzed transport of bis(glutathionato)cadmium

The yeast cadmium factor (YCF1) gene encodes an MgATP-energized glutathione S-conjugate transporter responsible for the vacuolar sequestration of organic compounds after their S-conjugation with glutathione. However, while YCF1 was originally isolated according to its ability to confer resistance to cadmium salts, neither its mode of interaction with Cd2+ nor the relationship between this process and organic glutathione-conjugate transport are known. Here we show through direct comparisons between vacuolar membrane vesicles purified from Saccharomyces cerevisiae strain DTY167, harboring a deletion of the YCF1 gene, and the isogenic wild-type strain DTY165 that YCF1 mediates the MgATP-energized vacuolar accumulation of Cd-glutathione complexes. The substrate requirements, kinetics and Cd2+/glutathione stoichiometry of cadmium uptake and the molecular weight of the transport-active complex demonstrate that YCF1 selectively catalyzes the transport of bis(glutathionato)cadmium (Cd x +GS2). On the basis of these results--the Cd2+ hypersensitivity of DTY167, versus DTY165, cells, the inducibility of YCF1-mediated transport, and the rapidity and spontaneity of Cd-GS2 formation--this new pathway is concluded to contribute substantially to Cd2+ detoxification.

CAX1, an H+/Ca2+ antiporter from Arabidopsis

Reestablishment of the resting state after stimulus-coupled elevations of cytosolic-free Ca2+ requires the rapid removal of Ca2+ from the cytosol of plant cells. Here we describe the isolation of two genes, CAX1 and CAX2, from Arabidopsis thaliana that suppress a mutant of Saccharomyces cerevisiae that has a defect in vacuolar Ca2+ accumulation. Both genes encode polypeptides showing sequence similarities to microbial H+/Ca2+ antiporters. Experiments on vacuolar membrane-enriched vesicles isolated from yeast expressing CAX1 or CAX2 demonstrate that these genes encode high efficiency and low efficiency H+/Ca2+ exchangers, respectively. The properties of the CAX1 gene product indicate that it is the high capacity transporter responsible for maintaining low cytosolic-free Ca2+ concentrations in plant cells by catalyzing pH gradient-energized vacuolar Ca2+ accumulation.

Isolation and characterization of cDNAs encoding vacuolar H(+)-pyrophosphatase isoforms from rice (Oryza sativa L.)

The vacuolar H(+)-pyrophosphatase (V-PPase) is an electrogenic H+ pump, which was found in the plant vacuolar membrane. Two cDNA clones (OVP1 and OVP2) encoding the V-PPase were isolated from cultured rice (Oryza sativa L.) cells and subsequently sequenced. The sequence analysis has revealed that OVP1 contains 2316 nucleotides of open reading frame (ORF) and 362 nucleotides of the 3'-untranslated region, whereas OVP2 comprises 2304 nucleotides of ORF and 312 nucleotides of the 3'-untranslated region. The nucleotide sequences of ORF of OVP1 and OVP2 are 80.7% identical, and their 5'- and 3'-untranslated regions have 39.4% and 48.4% identity, respectively. The polypeptides encoded by the ORF of OVP1 and OVP2 contain 771 and 767 amino acids, respectively, and the sequences of the OVP proteins are very similar to those of other V-PPases, which are shown to have 85-91% homology. Chromosomal mapping by RFLP techniques demonstrates that OVP1 and OVP2 are isoforms encoded by different genes. Both OVP1 and OVP2 are mapped on the same chromosome (chromosome 6) to a distance of ca.90 cM. Northern analysis indicates that the OVP1 and OVP2 are also expressed in intact rice plants and OVP2 shows higher expression in the calli than the roots and shoots, compared to OVP1. These results show that at least two genes encoding the V-PPases are present in rice genome and their expressions are probably regulated in a different manner.

PHYSIOLOGY OF ION TRANSPORT ACROSS THE TONOPLAST OF HIGHER PLANTS

The vacuole of plant cells plays an important role in the homeostasis of the cell. It is involved in the regulation of cytoplasmic pH, sequestration of toxic ions and xenobiotics, regulation of cell turgor, storage of amino acids, sugars and CO2 in the form of malate, and possibly as a source for elevating cytoplasmic calcium. All these activities are driven by two primary active transport mechanisms present in the vacuolar membrane (tonoplast). These two mechanisms employ high-energy metabolites to pump protons into the vacuole, establishing a proton electrochemical potential that mediates the transport of a diverse range of solutes. Within the past few years, great advances at the molecular and functional levels have been made on the characterization and identification of these mechanisms. The aim of this review is to summarize these studies in the context of the physiology of the plant cell.

Involvement of tyrosine residue in the inhibition of plant vacuolar H(+)-pyrophosphatase by tetranitromethane

Plant vacuolar vesicles contain a novel H(+)-translocating pyrophosphatase (H(+)-PPase, EC 3.6.1.1). Modification of tonoplast vesicles and purified vacuolar H(+)-PPase from etiolated mung bean seedlings with tetranitromethane (TNM) resulted in a progressive decline in H(+)-translocating pyrophosphatase activity. The half-maximal inhibition was brought about by 0.6, 1.0, and 0.8 mM TNM for purified and membrane-bound H(+)-PPases, and its associated proton translocation, respectively. The maximal inhibition of vacuolar H(+)-PPase by TNM occurred at a pH value above 8. Loss of activity of purified H(+)-pyrophosphatase followed pseudo-first order rate kinetics, yielding a first-order rate constant (k2) of 0.039 s(-1) and a steady-state dissociation constant of inactivation (Ki) of 0.02 mM. Covalent modification of vacuolar H(+)-PPase by TNM increased Km value of the enzyme for its substrate without a significant effect on Vmax. Double logarithmic plots of the pseudo-first order rate constant (kobs) versus TNM concentration exhibited a slope of 0.88, suggesting that at least one tyrosine residue was involved in the inactivation of H(+)-PPase enzymatic activity. Further spectrophotometric measurements of the nitrated H(+)-pyrophosphatase indicated that TNM could modify approximately two tyrosine residues/subunit of the enzyme. However, Tsou's analysis revealed that only one of those modified tyrosine residues directly participated in the inhibition of enzymatic activity of vacuolar H(+)-PPase. The physiological substrate, i.e., dimagnesium pyrophosphate, provided substantial protection against inactivation by TNM. Moreover, NEM pretreatment of the enzyme decreased the number of subsequent nitration of vacuolar H(+)-PPase. Taken together, we suggest that vacuolar H(+)-pyrophosphatase contains a substrate-protectable tyrosine residue conferring to the inhibition of its activity and this tyrosine residue may be located in a domain sensitive to the modification of Cys-634 by NEM.

The yeast cadmium factor protein (YCF1) is a vacuolar glutathione S-conjugate pump

The yeast cadmium factor gene (YCF1) from Saccharomyces cerevisiae, which was isolated according to its ability to confer cadmium resistance, encodes a 1,515 amino acid ATP-binding cassette (ABC) protein with extensive sequence homology to the human multidrug resistance-associated protein (MRP1) (Szczypka, M., Wemmie, J. A., Moye-Rowley, W. S., and Thiele, D. J. (1994) J. Biol. Chem. 269, 22853-22857). Direct comparisons between S. cerevisiae strain DTY167, harboring a deletion of the YCF1 gene, and the isogenic wild type strain, DTY165, demonstrate that YCF1 is required for increased resistance to the toxic effects of the exogenous glutathione S-conjugate precursor, 1-chloro-2,4-di-nitrobenzene, as well as cadmium. Whereas membrane vesicles isolated from DTY165 cells contain two major pathways for transport of the model compound S-(2,4-dinitrophenyl)glutathione (DNP-GS), an MgATP-dependent, uncoupler-insensitive pathway and an electrically driven pathway, the corresponding membrane fraction from DTY167 cells is more than 90% impaired for MgATP-dependent, uncoupler-insensitive DNP-GS transport. Of the two DNP-GS transport pathways identified, only the MgATP-dependent, uncoupler-insensive pathway is subject to inhibition by glutathione disulfide, vanadate, verapamil, and vinblastine. The capacity for MgATP-dependent, uncoupler-insensitive conjugate transport in vitro strictly copurifies with the acuolar membrane fraction. Intact DTY165 cells, but not DTY167 cells, mediate vacuolar accumulation of the quorescent glutathione-conjugate, monochlorobimane-GS. Introduction of plasmid borne, epitope-tagged gene encoding functional YCF1 into DTY167 cells alleviates the 1-chloro-2,4-dinitrobenzene-hypersensitive phenotype concomitant with restoration of the capacity of vacuolar membrane vesicles isolated from these cells for MgATP-dependent, uncoupler-insensitive DNP-GS transport. On the basis of these findings, the YCF1 gene of S. cerevisiae is inferred to encode an MgATP-energized, uncoupler-insensitive vacuolar glutathione S-conjugate transporter. The energy requirements, kinetics, substrate specificity, and inhibitor profile of YCF1-mediated transport demonstrate that the vacuolar glutathione conjugate pump of yeast bears a strong mechanistic resemblance to the MRP1-encoded transporter of mammalian cells and the cognate, but as yet molecularly undefined, function of plant cells.