Site-directed mutagenesis of the yeast PMA1 H(+)-ATPase. Structural and functional role of cysteine residues

Maleimide structure: a promising scaffold for the development of antimicrobial agents

Natural compounds bearing maleimide rings are a series of secondary metabolites derived from fungi/marine microorganisms, which are characterized by a general structure -CO-N(R)-CO-, and the R group is normally substituted with alkyl or aryl groups. Maleimide compounds show various biological activities such as antibacterial, antifungal, and anticancer activity. In this review, the broad-spectrum antimicrobial activities of 15 maleimide compounds from natural sources and 32 artificially synthesized maleimides were summarized, especially against Candida albicans, Sclerotinia sclerotiorum, and Staphylococcus aureus. It highlights that maleimide scaffold has tremendous potential to be utilized in the development of novel antimicrobial agents.

A conserved, buried cysteine near the P-site is accessible to cysteine modifications and increases ROS stability in the P-type plasma membrane H+-ATPase

Sulfur-containing amino acid residues function in antioxidative responses, which can be induced by the reactive oxygen species generated by excessive copper and hydrogen peroxide. In all Na+/K+, Ca2+, and H+ pumping P-type ATPases, a cysteine residue is present two residues upstream of the essential aspartate residue, which is obligatorily phosphorylated in each catalytic cycle. Despite its conservation, the function of this cysteine residue was hitherto unknown. In this study, we analyzed the function of the corresponding cysteine residue (Cys-327) in the autoinhibited plasma membrane H+-ATPase isoform 2 (AHA2) from Arabidopsis thaliana by mutagenesis and heterologous expression in a yeast host. Enzyme kinetics of alanine, serine, and leucine substitutions were identical with those of the wild-type pump but the sensitivity of the mutant pumps was increased towards copper and hydrogen peroxide. Peptide identification and sequencing by mass spectrometry demonstrated that Cys-327 was prone to oxidation. These data suggest that Cys-327 functions as a protective residue in the plasma membrane H+-ATPase, and possibly in other P-type ATPases as well.

Functioning of Yeast Pma1 H+-ATPase under Changing Charge: Role of Asp739 and Arg811 Residues

The plasma membrane Pma1 H+-ATPase of the yeast Saccharomyces cerevisiae contains conserved residue Asp739 located at the interface of transmembrane segment M6 and the cytosol. Its replacement by Asn or Val (Petrov et al. (2000) J. Biol. Chem., 275, 15709-15716) or by Ala (Miranda et al. (2011) Biochim. Biophys. Acta, 1808, 1781-1789) caused complete blockage of biogenesis of the enzyme, which did not reach secretory vesicles. It was proposed that a strong ionic bond (salt bridge) could be formed between this residue and positively charged residue(s) in close proximity, and the replacement D739A disrupted this bond. Based on a 3D homology model of the enzyme, it was suggested that the conserved Arg811 located in close proximity to Asp739 could be such stabilizing residue. To test this suggestion, single mutants with substituted Asp739 (D739V, D739N, D739A, and D739R) and Arg811 (R811L, R811M, R811A, and R811D) as well as double mutants carrying charge-neutralizing (D739A/R811A) or charge-swapping (D739R/R811D) substitutions were used. Expression of ATPases with single substitutions R811A and R811D were 38-63%, and their activities were 29-30% of the wild type level; ATP hydrolysis and H+ transport in these enzymes were essentially uncoupled. For the other substitutions including the double mutations, the biogenesis of the enzyme was practically blocked. These data confirm the important role of Asp739 and Arg811 residues for the biogenesis and function of the enzyme, suggesting their importance for defining H+ transport determinants but ruling out, however, the existence of a strong ionic bond (salt bridge) between these two residues and/or importance of such bridge for structure-function relationships in Pma1 H+-ATPase.

[Effect of point substitutions of Asp-714 and Asp-720 residues on the structure and function of the H+ -ATPase of the yeast plasma membrane]

Membrane-spanning M5 and M6 segments, which play a role in the formation of cation transport sites in H(+)-, Ca2(+)-, K(+)-, Na(+)-, and other P2-ATPases, are connected by a short extracytoplasmic loop. In the yeast plasma membrane H(+)-ATPase, which belongs to a family of P2-ATPases, the loop is connected to M5 and M6 through the Asp-714 and Asp-720 residues. In this work, the effect of point amino, acidreplacements of Asp-714 and Asp-720 by Ala, Val, Asn, and Glu residues on the function of the enzyme was studied. The Asp714Asn point mutant possessed activities similar to those of the wild-type enzyme, whereas the replacement of Asp-714 by other amino acid residues disrupted biogenesis and led to a loss of activity. All mutants with substitution of Asp-720 were expressed and possessed relatively high activity. The D720V mutant displayed significantly reduced expression levels, activity, H+ transport, and ATP hydrolyzing activity. Thus, substitutions of Asp-714, except for the D714N mutant, led to significant defects in biogenesis and/or function of the enzyme. The results indicate the important role for the Asp-714 residue in biogenesis, structure stability, and enzyme function.

Role of loop L5-6 connecting transmembrane segments M5 and M6 in biogenesis and functioning of yeast Pma1 H+-ATPase

The L5-6 loop is a short extracytoplasmic stretch (714-DNSLDID) connecting transmembrane segments M5 and M6 and forming along with segments M4 and M8 the core through which cations are transported by H+-, Ca2+-, K+,Na+-, H+,K+-, and other P2-ATPases. To study structure-function relationships within this loop of the yeast plasma membrane Pma1 H+-ATPase, alanine- and cysteine-scanning mutagenesis has been employed. Ala and Cys substitutions for the most conserved residue (Leu717) led to complete block in biogenesis preventing the enzyme from reaching secretory vesicles. The Ala replacement at Asp714 led to five-fold decrease in the mutant expression and loss of its activity, while the Cys substitution blocked biogenesis completely. Replacements of other residues did not lead to loss of enzymatic activity. Additional replacements were made for Asp714 and Asp720 (Asp®Asn/Glu). Of the substitutions made at Asp714, only D714N partially restored the mutant enzyme biogenesis and functioning. However, all mutant enzymes with substituted Asp720 were active. The expressed mutants (34-95% of the wild-type level) showed activity high enough (35-108%) to be analyzed in detail. One of the mutants (I719A) had three-fold reduced coupling ratio between ATP hydrolysis and H+ transport; however, the I719C mutation was rather indistinguishable from the wild-type enzyme. Thus, substitutions at two of the seven positions seriously affected biogenesis and/or functioning of the enzyme. Taken together, these results suggest that the M5-M6 loop residues play an important role in protein stability and function, and they are probably responsible for proper arrangement of transmembrane segments M5 and M6 and other domains of the enzyme. This might also be important for the regulation of the enzyme.

Point mutations in the extracytosolic loop between transmembrane segments M5 and M6 of the yeast Pma1 H+-ATPase: alanine-scanning mutagenesis

Membrane-spanning segments M4, M5, M6, and M8 of the H(+)-, Ca(2+)-, and K(+), Na(+)-ATPases, which belong to the P2-type pumps are the core through which cations are transported. M5 and M6 loop is a short extracytoplasmic stretch of the seven amino acid residues (714-DNSLDID) connecting two of these segments, M5 and M6, where residues involved in the formation of the proton-binding site(s) are located. In the present study, we have used alanine-scanning mutagenesis to explore the structural and functional relationships within this loop of the yeast plasma membrane Pma1 H(+)-ATPase. Of the 7 Ala mutants made, substitution for the most conserved residue (Leu-717) has led to a severe misfolding and complete block in biogenesis of the mutant enzyme. The replacement of Asp-714 has also caused misfolding leading to significant decrease in the expression of the mutant and loss of activity. The remaining mutants were expressed in secretory vesicles at 21-119% of the wild-type level and were active enough to be analyzed in detail. One of these mutants (I719A) showed five- to threefold decrease in both expression and ATP hydrolyzing and H(+) pumping activities and also threefold reduction in the coupling ratio between ATP hydrolysis and H(+) transport. Thus, Ala substitutions at three positions of the seven seriously affected biogenesis, folding, stability and/or functioning of the enzyme. Taken together, these results lead to suggestion that M5 and M6 loop play an important role in the protein stability and function and is responsible for proper arrangement of transmembrane segments M5 and M6 and probably other domains of the enzyme. Results for additional conserved substitutions (Asn and Glu) at Asp-714 and Asp-720 confirmed this suggestion.

Structure-function relationships in membrane segment 6 of the yeast plasma membrane Pma1 H(+)-ATPase

The crystal structures of the Ca(2+)- and H(+)-ATPases shed light into the membrane embedded domains involved in binding and ion translocation. Consistent with site-directed mutagenesis, these structures provided additional evidence that membrane-spanning segments M4, M5, M6 and M8 are the core through which cations are pumped. In the present study, we have used alanine/serine scanning mutagenesis to study the structure-function relationships within M6 (Leu-721-Pro-742) of the yeast plasma membrane ATPase. Of the 22 mutants expressed and analyzed in secretory vesicles, alanine substitutions at two well conserved residues (Asp-730 and Asp-739) led to a complete block in biogenesis; in the mammalian P-ATPases, residues corresponding to Asp-730 are part of the cation-binding domain. Two other mutants (V723A and I736A) displayed a dramatic 20-fold increase in the IC(50) for inorganic orthovanadate compared to the wild-type control, accompanied by a significant reduction in the K(m) for Mg-ATP, and an alkaline shift in the pH optimum for ATP hydrolysis. This behavior is apparently due to a shift in equilibrium from the E(2) conformation of the ATPase towards the E(1) conformation. By contrast, the most striking mutants lying toward the extracellular side in a helical structure (L721A, I722A, F724A, I725A, I727A and F728A) were expressed in secretory vesicles but had a severe reduction of ATPase activity. Moreover, all of these mutants but one (F728A) were unable to support yeast growth when the wild-type chromosomal PMA1 gene was replaced by the mutant allele. Surprisingly, in contrast to M8 where mutations S800A and E803Q (Guerra et al., Biochim. Biophys. Acta 1768: 2383-2392, 2007) led to a dramatic increase in the apparent stoichiometry of H(+) transport, three substitutions (A726S, A732S and T733A) in M6 showed a reduction in the apparent coupling ratio. Taken together, these results suggest that M6 residues play an important role in protein stability and function, and probably are responsible for cation binding and stoichiometry of ion transport as suggested by homology modeling.

Functioning of Saccharomyces cerevisiae Pma1 H+-ATPase carrying the minimal number of cysteine residues

Pma1 H+-ATPase is the primary proton pump in the plasma membrane of the yeast Saccharomyces cerevisiae. It generates an electrochemical proton gradient, thus providing energy for secondary solute transport systems. The enzyme contains nine cysteines, three (Cys148, Cys312, and Cys867) in transmembrane segments and the rest (Cys221, Cys376, Cys409, Cys472, Cys532, and Cys569) in the cytosolic parts of the molecule. Although individually they are not essential for the functioning of the ATPase, substitution of all of them leads to the loss of enzyme activity and impairment of biogenesis. By means of site-directed mutagenesis combined with other molecular-biological and biochemical methods, this work defines different combinations of minimal cysteine content that are consistent with ATPase function.

Evaluation of the antimicrobial activity of ebselen: Role of the yeast plasma membrane H+-ATPase

Ebselen (2-phenyl-1,2-benzisoselenazol-3(2H)-one) is a selenium-containing antioxidant demonstrating anti-inflammatory and cytoprotective properties in mammalian cells and cytotoxicity in lower organisms. The mechanism underlying the antimicrobial activity of ebselen remains unclear. It has recently been proposed that, in lower organisms like yeast, the plasma membrane H+-ATPase (Pma1p) could serve as a potential target for this synthetic organoselenium compound. Using yeast and bacteria, the present study found ebselen to inhibit microbial growth in a concentration- and time-dependent manner, and yeast and Gram-positive bacteria to be more sensitive to this action (IC50 approximately 2-5 microM) than Gram-negative bacteria (IC50 < 80 microM). Washout experiments and scanning electron microscopic analysis revealed ebselen to possess fungicidal activity. In addition, ebselen was found to inhibit medium acidification by PMA1-proficient haploid yeast in a concentration-dependent manner. Additional studies comparing PMA1 (+/-) and PMA1 (+/+) diploid yeast cells revealed the mutant to be more sensitive to treatment with ebselen than the wild type. Ebselen also inhibited the ATPase activity of Pma1p from S. cerevisiae in a concentration-dependent manner. The interaction of ebselen with the sulfhydryl-containing compounds L-cysteine and reduced glutathione resulted in the complete and partial prevention, respectively, of the inhibition of Pma1p ATPase activity by ebselen. Taken together, these results suggest that the fungicidal action of ebselen is due, at least in part, to interference with both the proton-translocating function and the ATPase activity of the plasma membrane H+-ATPase.

Role of strategic cysteine residues in oxidative damage to the yeast plasma membrane H(+)-ATPase caused by Fe- and Cu-containing Fenton reagents

Damage caused to Saccharomyces cerevisiae SY4 plasma membrane H(+)-ATPase by Fe- and Cu-Fenton reagents was determined in secretory vesicles containing enzyme in which Cys residues were replaced singly or in pairs by Ala. Cys-221 situated in a beta-sheet domain between M2 and M3 segments, phosphorylation domain-located Cys-409 and Cys-532 situated at the ATP-binding site play a role in the inactivation. In the presence of all three residues the enzyme exhibited a certain basic inactivation, which did not change when Cys-532 was replaced with Ala. In mutants having intact Cys-532 but lacking one or both other cysteines, replacement of Cys-221 with Ala led to lower inactivation, suggesting that Cys-221 may serve as a target for metal-catalyzed oxidation and intact Cys-532 promotes this target role of Cys-221. In contrast, the absence of Cys-409 caused higher inactivation by Fe-Fenton. Cys-532 thus seems to serve as a target for Fe-Fenton, intact Cys-409 causing a conformational change that makes Cys-532 less accessible to oxidation. The mutant lacking both Cys-221 and Cys-409 is more sensitive to Fe-Fenton than to Cu-Fenton and the absence of both Cys residues thus seems to expose presumable extra Fe-binding sites. These data and those on protection by ATP, ADP, 1,4-dithiothreitol and deferrioxamine B point to complex interactions between individual parts of the enzyme molecule that determine its sensitivity towards Fenton reagents. ATPase fragmentation caused by the two reagents differed in that the Fe-Fenton reagent produced in Western blot "smears" whereas the Cu-Fenton reagent produced defined fragments.

HOCl-mediated cell death and metabolic dysfunction in the yeast Saccharomyces cerevisiae

The nature of oxidative damage to Saccharomyces cerevisiae caused by levels of HOCl that inhibit cell replication was explored with the intent of identifying the loci of lethal lesions. Functions of cytosolic enzymes and organelles that are highly sensitive to inactivation by HOCl, including aldolase, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and the mitochondrion, were only marginally affected by exposure of the yeast to levels of HOCl that completely inhibited colony formation. Loss of function in membrane-localized proteins, including the hexose transporters and PMA1 H(+)-ATPase, which is the primary proton pump located within the S. cerevisiae plasma membrane, was also marginal and K(+) leak rates to the extracellular medium increased only slowly with exposure to increasing amounts of HOCl, indicating that the plasma membrane retained its intrinsic impermeability to ions and metabolites. Adenylate phosphorylation levels in fermenting yeast declined in parallel with viability; however, yeast grown on respiratory substrates maintained near-normal phosphorylation levels at HOCl doses several-fold greater than that required for killing. This overall pattern of cellular response to HOCl differs markedly from that previously reported for bacteria, which appear to be killed by inhibition of plasma membrane proteins involved in energy transduction. The absence of significant loss of function in critical oxidant-sensitive cellular components and retention of ATP-synthesizing capabilities in respiring yeast cells exposed to lethal levels of HOCl suggests that toxicity in this case may arise by programmed cell death.

Substrate recognition in ER-associated degradation mediated by Eps1, a member of the protein disulfide isomerase family

Pma1-D378N is a misfolded plasma membrane protein in yeast that is prevented from delivery to the cell surface and targeted instead for ER-associated degradation (ERAD). Degradation of Pma1-D378N is dependent on the ubiquitin ligase Doa10 and the ubiquitin chaperone Cdc48. Recognition of Pma1-D378N by the ERAD pathway is dependent on Eps1, a transmembrane member of the protein disulfide isomerase (PDI) oxidoreductase family. Eps1 has two thioredoxin-like domains containing a CPHC and a CDKC active site. Although Eps1 interaction with wild-type Pma1 was not detected, Eps1 co-immunoprecipitates with Pma1-D378N. Eps1 interaction with Pma1-D378N requires the CPHC motif, although both thioredoxin-like domains appear to cooperate in substrate recognition. In the absence of the native transmembrane domain and cytoplasmic tail of Eps1, degradation of Pma1-D378N is slowed, suggesting that Eps1 facilitates presentation of substrate to membrane-bound components of the degradation machinery. Genetic interactions with other mutants of the ERAD machinery and induction of the unfolded protein response in eps1Delta cells support a general role for Eps1 as a recognition component of the ERAD pathway.

Modes of antifungal action of (2E)-alkenals against Saccharomyces cerevisiae

A series of aliphatic (2E)-alkenals from C(5) to C(14) were tested for their antifungal activity against Saccharomyces cerevisiae ATCC 7754. (2E)-Undecenal (C(11)) was found to be the most effective with the minimum fungicidal concentration (MFC) of 6.25 microgram/mL, followed by (2E)-decenal (C(10)) with an MFC of 12.5 microgram/mL. The time-kill curve study showed that (2E)-undecenal was fungicidal against S. cerevisiae at any growth stage, and this activity was not influenced by pH values. The (2E)-alkenals inhibited glucose-induced acidification by inhibiting the plasma membrane H(+)-ATPase. The primary antifungal action of medium-chain (C(9)-C(12)) (2E)-alkenals against S. cerevisiae comes from their ability to function as nonionic surface-active agents (surfactants), disrupting the native membrane-associated function nonspecifically. Hence, the antifungal activity of (2E)-alkenals is mediated by biophysical processes, and the maximum activity can be obtained when the balance between the hydrophilic and hydrophobic portions becomes the most appropriate.

Modeling the three-dimensional structure of H+-ATPase of Neurospora crassa

Homology modeling in combination with transmembrane topology predictions are used to build the atomic model of Neurospora crassa plasma membrane H+-ATPase, using as template the 2.6 A crystal structure of rabbit sarcoplasmic reticulum Ca2+-ATPase [Toyoshima, C., Nakasako, M., Nomura, H. & Ogawa, H. (2000) Nature 405, 647-655]. Comparison of the two calcium-binding sites in the crystal structure of Ca2+-ATPase with the equivalent region in the H+-ATPase model shows that the latter is devoid of most of the negatively charged groups required to bind the cations, suggesting a different role for this region. Using the built model, a pathway for proton transport is then proposed from computed locations of internal polar cavities, large enough to contain at least one water molecule. As a control, the same approach is applied to the high-resolution crystal structure of halorhodopsin and the proton pump bacteriorhodopsin. This revealed a striking correspondence between the positions of internal polar cavities, those of crystallographic water molecules and, in the case of bacteriorhodopsin, the residues mediating proton translocation. In our H+-ATPase model, most of these cavities are in contact with residues previously shown to affect coupling of proton translocation to ATP hydrolysis. A string of six polar cavities identified in the cytoplasmic domain, the most accurate part of the model, suggests a proton entry path starting close to the phosphorylation site. Strikingly, members of the haloacid dehalogenase superfamily, which are close structural homologs of this domain but do not share the same function, display only one polar cavity in the vicinity of the conserved catalytic Asp residue.

Stalk segment 5 of the yeast plasma membrane H(+)-ATPase. Labeling with a fluorescent maleimide reveals a conformational change during glucose activation

Glucose is well known to cause a rapid, reversible activation of the yeast plasma membrane H(+)-ATPase, very likely mediated by phosphorylation of two or more Ser/Thr residues near the C terminus. Recent mutagenesis studies have shown that glucose-dependent activation can be mimicked constitutively by amino acid substitutions in stalk segment 5 (S5), an alpha-helical stretch connecting the catalytic part of the ATPase with transmembrane segment 5 (Miranda, M., Allen, K. E., Pardo, J. P., and Slayman, C. W. (2001) J. Biol. Chem. 276, 22485-22490). In the present work, the fluorescent maleimide Alexa-488 has served as a probe for glucose-dependent changes in the conformation of S5. Experiments were carried out in a "3C" version of the ATPase, from which six of nine native cysteines had been removed by site-directed mutagenesis to eliminate background labeling by Alexa-488. In this construct, three of twelve cysteines introduced at various positions along S5 (A668C, S672C, and D676C) reacted with the Alexa dye in a glucose-independent manner, as shown by fluorescent labeling of the 100 kDa Pma1 polypeptide and by isolation and identification of the corresponding tryptic peptides. Especially significant was the fact that three additional cysteines reacted with Alexa-488 more rapidly (Y689C) or only (V665C and L678C) in plasma membranes from glucose-metabolizing cells. The results support a model in which the S5 alpha-helix undergoes a significant change in conformation to expose positions 665, 678, and 689 during glucose-dependent activation of the ATPase.

Antifungal mechanism of polygodial

The primary antifungal action of polygodial comes in part from its ability to function as a nonionic surfactant, disrupting the lipid-protein interface of integral proteins and denaturing their conformation. As a result, the antifungal mechanism of this sesquiterpene dialdehyde is associated with the membrane functions or derangement of the membrane. For example, the glucose-induced medium acidification process of Saccharomyces cerevisiae was inhibited by polygodial, presumably caused by inhibition of the plasma membrane H(+)-ATPase. However, the potent antifungal activity of polygodial results from its multiple functions.

Altered distribution of the yeast plasma membrane H+-ATPase as a feature of vacuolar H+-ATPase null mutants

The effect of vacuolar H(+)-ATPase (V-ATPase) null mutations on the targeting of the plasma membrane H(+)-ATPase (Pma1p) through the secretory pathway was analyzed. Gas1p, which is another plasma membrane component, was used as a control for the experiments with Pma1p. Contrary to Gas1p, which is not affected by the deletion of the V-ATPase complex in the V-ATPase null mutants, the amount of Pma1p in the plasma membrane is markedly reduced, and there is a large accumulation of the protein in the endoplasmic reticulum. Kex2p and Gef1p, which are considered to reside in the post-Golgi vesicles, were suggested as required for the V-ATPase function; hence, their null mutant phenotype should have been similar to the V-ATPase null mutants. We show that, in addition to the known differences between those yeast phenotypes, deletions of KEX2 or GEF1 in yeast do not affect the distribution of Pma1p as the V-ATPase null mutant does. The possible location of the vital site of acidification by V-ATPase along the secretory pathway is discussed.

Expression of an active Na,K-ATPase with an alpha-subunit lacking all twenty-three native cysteine residues

We have constructed a mutant Na,K-ATPase alpha1-subunit with all native cysteine residues replaced. Using the baculovirus system, this cysteine-less alpha1-subunit and wild-type beta1-subunit were expressed in High Five cells. After 3 days of infection, cells were fractionated, and endoplasmic reticulum, Golgi apparatus, and plasma membranes were isolated. The molecular activity of the cysteine-less mutant in the plasma membranes was close to the wild-type protein (8223 min(-)(1) versus 6655 min(-)(1)). Cation and ATP activation of Na,K-ATPase activities revealed that replacing all 23 cysteines resulted in only a 50% reduction of K(m) for Na(+), a 2-fold increase in K(m) for K(+), and no changes in K(m) for ATP. The distribution of alpha-subunits among the membranes showed a high percentage of cysteine-less protein in the endoplasmic reticulum and Golgi apparatus compared with the wild-type protein. Furthermore, the cellular stability of the alphabeta assembly appeared reduced in the cysteine-less mutant. Cells harvested after more than 3 days of infection showed extensive degradation of the cysteine-less alpha-subunit, which is not observed with the wild-type enzyme. Thus the Na,K-ATPase contains no cysteine residues that are critical for function, but the folding and/or assembly pathway of this enzyme is affected by total cysteine substitution.

Functional role of charged residues in the transmembrane segments of the yeast plasma membrane H+-ATPase

As defined by hydropathy analysis, the membrane-spanning segments of the yeast plasma membrane H(+)-ATPase contain seven negatively charged amino acids (Asp and Glu) and four positively charged amino acids (Arg and His). To explore the functional role of these residues, site-directed mutants at all 11 positions and at Glu-288, located near the cytoplasmic end of M3, have been constructed and expressed in yeast secretory vesicles. Substitutions at four of the positions (Glu-129, Glu-288, Asp-833, and Arg-857) had no significant effect on ATP hydrolysis or ATP-dependent proton pumping, substitutions at five additional positions (Arg-695, His-701, Asp-730, Asp-739, and Arg-811) led to misfolding of the ATPase and blockage at an early stage of biogenesis, and substitutions of Asp-143 allowed measurable biogenesis but nearly abolished ATP hydrolysis and proton transport. Of greatest interest were mutations of Glu-703 in M5 and Glu-803 in M8, which altered the apparent coupling between hydrolysis and transport. Three Glu-703 mutants (E703Q, E703L, E703D) showed significantly reduced pumping over a wide range of hydrolysis values and thus appeared to be partially uncoupled. At Glu-803, by contrast, one mutant (E803N) was almost completely uncoupled, while another (E803Q) pumped protons at an enhanced rate relative to the rate of ATP hydrolysis. Both Glu-703 and Glu-803 occupy positions at which amino acid substitutions have been shown to affect transport by mammalian P-ATPases. Taken together, the results provide growing evidence that residues in membrane segments 5 and 8 of the P-ATPases contribute to the cation transport pathway and that the fundamental mechanism of transport has been conserved throughout the group.

Functional role of cysteine residues in the (Na,K)-ATPase alpha subunit

The structural-functional roles of 23 cysteines present in the sheep (Na,K)-ATPase alpha1 subunit were studied using site directed mutagenesis, expression, and kinetics analysis. Twenty of these cysteines were individually substituted by alanine or serine. Cys452, Cys455 and Cys456 were simultaneously replaced by serine. These substitutions were introduced into an ouabain resistant alpha1 sheep isoform and expressed in HeLa cells under ouabain selective pressure. HeLa cells transfected with a cDNA encoding for replacements of Cys242 did not survive ouabain selective pressure. Single substitutions of the remaining cysteines yielded functional enzymes, although some had reduced turnover rates. Only minor variations were observed in the enzyme Na(+) and K(+) dependence as a result of these replacements. Some substitutions apparently affect the E1<-->E2 equilibrium as suggested by changes in the K(m) of ATP acting at its low affinity binding site. These results indicate that individual cysteines, with the exception of Cys242, are not essential for enzyme function. Furthermore, this suggests that the presence of putative disulfide bridges is not required for alpha1 subunit folding and subsequent activity. A (Na,K)-ATPase lacking cysteine residues in the transmembrane region was constructed (Cys104, 138, 336, 802, 911, 930, 964, 983Xxx). No alteration in the K(1/2) of Na(+) or K(+) for (Na,K)-ATPase activation was observed in the resulting enzyme, although it showed a 50% reduction in turnover rate. ATP binding at the high affinity site was not affected. However, a displacement in the E1<-->E2 equilibrium toward the E1 form was indicated by a small decrease in the K(m) of ATP at the low affinity site accompanied by an increase in IC(50) for vanadate inhibition. Thus, the transmembrane cysteine-deficient (Na,K)-ATPase appears functional with no critical alteration in its interactions with physiological ligands.

Determining the structure and mechanism of the human multidrug resistance P-glycoprotein using cysteine-scanning mutagenesis and thiol-modification techniques

The multidrug resistance P-glycoprotein is an ATP-dependent drug pump that extrudes a broad range of hydrophobic compounds out of cells. Its physiological role is likely to protect us from exogenous and endogenous toxins. The protein is important because it contributes to the phenomenon of multidrug resistance during AIDS and cancer chemotherapy. We have used cysteine-scanning mutagenesis and thiol-modification techniques to map the topology of the protein, show that both nucleotide-binding domains are essential for activity, examine packing of the transmembrane segments, map the drug-binding site, and show that there is cross-talk between the ATP-binding sites and the transmembrane segments.

Evidence for a salt bridge between transmembrane segments 5 and 6 of the yeast plasma-membrane H+-ATPase

The plasma-membrane H+-ATPase of Saccharomyces cerevisiae, which belongs to the P2 subgroup of cation-transporting ATPases, is encoded by the PMA1 gene and functions physiologically to pump protons out of the cell. This study has focused on hydrophobic transmembrane segments M5 and M6 of the H+-ATPase. In particular, a conserved aspartate residue near the middle of M6 has been found to play a critical role in the structure and biogenesis of the ATPase. Site-directed mutants in which Asp-730 was replaced by an uncharged residue (Asn or Val) were abnormally sensitive to trypsin, consistent with the idea that the proteins were poorly folded, and immunofluorescence confocal microscopy showed them to be arrested in the endoplasmic reticulum. Similar defects are known to occur when either Arg-695 or His-701 in M5 is replaced by a neutral residue (Dutra, M. B., Ambesi, A., and Slayman, C. W. (1998) J. Biol. Chem. 273, 17411-17417). To search for possible charge-charge interactions between Asp-730 and Arg-695 or His-701, double mutants were constructed in which positively and negatively charged residues were swapped or eliminated. Strikingly, two of the double mutants (R695D/D730R and R695A/D730A) regained the capacity for normal biogenesis and displayed near-normal rates of ATP hydrolysis and ATP-dependent H+ pumping. These results demonstrate that neither Arg-695 nor Asp-730 is required for enzymatic activity or proton transport, but suggest that there is a salt bridge between the two residues, linking M5 and M6 of the 100-kDa polypeptide.

Phosphorylation region of the yeast plasma-membrane H+-ATPase. Role in protein folding and biogenesis

Mutations at the phosphorylation site (Asp-378) of the yeast plasma-membrane H+-ATPase have been shown previously to cause misfolding of the ATPase, preventing normal movement along the secretory pathway; Asp-378 mutations also block the biogenesis of co-expressed wild-type ATPase and lead to a dominant lethal phenotype. To ask whether these defects are specific for Asp-378 or whether the phosphorylation region as a whole is involved, alanine-scanning mutagenesis has been carried out to examine the role of 11 conserved residues flanking Asp-378. In the sec6-4 expression system (Nakamoto, R. K., Rao, R., and Slayman, C. W. (1991) J. Biol. Chem. 266, 7940-7949), the mutant ATPases displayed varying abilities to reach the secretory vesicles that deliver plasma-membrane proteins to the cell surface. Indirect immunofluorescence of intact cells also gave evidence for a spectrum of behavior, ranging from mutant ATPases completely arrested (D378A, K379A, T380A, and T384A) or partially arrested in the endoplasmic reticulum to those that reached the plasma membrane in normal amounts (C376A, S377A, and G381A). Although the extent of ER retention varied among the mutants, the endoplasmic reticulum appeared to be the only secretory compartment in which the mutant ATPases accumulated. All of the mutant proteins that localized either partially or fully to the ER were also malfolded based on their abnormal sensitivity to trypsin. Among them, the severely affected mutants had a dominant lethal phenotype, and even the intermediate mutants caused a visible slowing of growth when co-expressed with wild-type ATPase. The effects on growth could be traced to the trapping of the wild-type enzyme with the mutant enzyme in the ER, as visualized by double label immunofluorescence. Taken together, the results indicate that the residues surrounding Asp-378 are critically important for ATPase maturation and transport to the cell surface.

The essential Aspergillus nidulans gene pmaA encodes an homologue of fungal plasma membrane H(+)-ATPases

pmaA, an Aspergillus nidulans gene encoding a P-ATPase, has been cloned by heterologous hybridization with the yeast PMA1 gene. The putative 990-residue PmaA polypeptide shows 50% identity to Saccharomyces cerevisiae and Neurospora crassa plasma membrane H(+)-ATPases and weak (19-26%) identity to other yeast P-type cation-translocating ATPases. PmaA contains all catalytic domains characterizing H(+)-ATPases. pmaA transcript levels are not regulated by PacC, the transcription factor mediating pH regulation, and were not significantly affected by an extreme creAd mutation resulting in carbon catabolite derepression. Deletion of pmaA causes lethality, but a single copy of the gene is sufficient to support normal growth rate in pmaA hemizygous diploids, even under acidic growth conditions. As compared to other fungal H(+)-ATPases, PmaA presents three insertions, 39, 7, and 16 residues long, in the conserved central region of the protein. Two of these insertions are predicted to be located in extracellular loops and might be of diagnostic value for the identification of Aspergillus species. Their absence from most mammalian P-type ATPases may have implications for antifungal therapy.

Structure of yeast plasma membrane H(+)-ATPase: comparison of activated and basal-level enzyme forms

Plasma membrane H(+)-ATPase of the yeast Saccharomyces cerevisiae was isolated and purified in its two forms, the activated A-ATPase from glucose-metabolising cells, and the basal-level B-ATPase from cells with endogenous metabolism only. Structure of the two enzyme forms and the effects of beta, gamma-imidoadenosine 5'-triphosphate (AMP-PNP) and of diethylstilbestrol (DES) thereon were analysed by FT-IR spectroscopy. IR spectra revealed the presence of two populations of alpha-helices with different exposure to the solvent in both the A-ATPase and B-ATPase. AMP-PNP did not affect the secondary structure of A-ATPase while DES affected the ratio of the two alpha-helix populations. Thermal denaturation experiments suggested a more stable structure in the B-form than in the A-form. AMP-PNP stabilised the A-ATPase structure while DES destabilised both enzyme forms. IR spectra showed that 60% of the amide hydrogens were exchanged for deuterium in both forms at 20 degrees C. The remaining 40% were exchanged at higher temperatures. The maximum amount of H/D exchange was observed at 50-55 degrees C for both enzyme forms, while in the presence of DES it was observed at lower temperatures. The data do not contradict the possibility that the activation of H(+)-ATPase is due to the C-terminus of the enzyme dissociating from the ATP-binding site which is covered by it in the less active form.

Properties of a cysteine-free proton-pumping nicotinamide nucleotide transhydrogenase

Nicotinamide nucleotide transhydrogenase from Escherichia coli was investigated with respect to the roles of its cysteine residues. This enzyme contains seven cysteines, of which five are located in the alpha subunit and two are in the beta subunit. All cysteines were replaced by site-directed mutagenesis. The final construct (alphaC292T, alphaC339T, alphaC395S, alphaC397T, alphaC435S, betaC147S, betaC260S) was inserted normally in the membrane and underwent the normal NADPH-dependent conformational change of the beta subunit to a trypsin-sensitive state. Reduction of NADP+ by NADH driven by ATP hydrolysis or respiration was between 32% and 65% of the corresponding wild-type activities. Likewise, the catalytic and proton pumping activities of the purified cysteine-free enzyme were at least 30% of the purified wild-type enzyme activities. The H+/H- ratio for both enzymes was 0.5, although the cysteine-free enzyme appeared to be more stable than the wild-type enzyme in proteoliposomes. No bound NADP(H) was detected in the enzymes. Modification of transhydrogenase by diethyl pyrocarbonate and the subsequent inhibition of the enzyme were unaffected by removal of the cysteines, indicating a lack of involvement of cysteines in this process. Replacement of cysteine residues in the alpha subunit resulted in no or little change in activity, suggesting that the basis for the decreased activity was probably the modification of the conserved beta-subunit residue Cys-260 or (less likely) the non-conserved beta-subunit residue Cys-147. It is concluded that the cysteine-free transhydrogenase is structurally and mechanistically very similar to the wild-type enzyme, with minor modifications of the properties of the NADP(H) site, possibly mediated by the betaC260S mutation. The cysteine-free construct will be a valuable tool for studying structure-function relationships of transhydrogenases.

Reactive cysteines of the yeast plasma-membrane H+-ATPase (PMA1). Mapping the sites of inactivation by N-ethylmaleimide

We have taken advantage of cysteine mutants described previously (Petrov, V. V., and Slayman, C. W. (1995) J. Biol. Chem. 270, 28535-28540) to map the sites at which N-ethylmaleimide (NEM) reacts with the plasma-membrane H+ATPase (PMA)1 of Saccharomyces cerevisiae. When membrane vesicles containing the ATPase were incubated with NEM, six of nine mutants with single cysteine substitutions showed sensitivity similar to the wild-type enzyme. By contrast, C221A and C532A were inactivated more slowly than the wild-type control, and the C221, 532A double mutant was completely resistant, indicating that Cys-221 and Cys-532 are NEM-reactive residues. In the presence of 10 mM MgADP, the wild-type ATPase was partially protected against NEM; parallel experiments with the C221A and C532A mutants showed that the protection occurred at Cys-532, located in or near the nucleotide-binding site. Unexpectedly, the inactivation of the C409A ATPase was approximately 4-fold more rapid than in the case of the wild-type enzyme. Experiments with double mutants made it clear that this resulted from an acidic shift in pKa and a consequent acceleration of the reaction rate at Cys-532. One simple interpretation is that substitution of Cys-409 leads to a local conformational change within the central hydrophilic domain. Consistent with this idea, the reaction of fluorescein 5'-isothiocyanate at Lys-474 was also stimulated approximately 3. 5-fold by the C409A mutation. Taken together, the results of this study provide new information about the reactivity of individual Cys residues within the ATPase and pave the way to tag specific sites for structural and functional studies of the enzyme.