Monomers of the Neurospora plasma membrane H+-ATPase catalyze efficient proton translocation

Structure, function and biogenesis of the fungal proton pump Pma1

The fungal plasma membrane proton pump Pma1 is an integral plasma membrane protein of the P-type ATPase family. It is an essential enzyme responsible for maintaining a constant cytosolic pH and for energising the plasma membrane to secondary transport processes. Due to its importance for fungal survival and absence from animals, Pma1 is also a highly sought-after drug target. Until recently, its characterisation has been limited to functional, mutational and localisation studies, due to a lack of high-resolution structural information. The determination of three cryo-EM structures of Pma1 in its unique hexameric state offers a new level of understanding the molecular mechanisms underlying the protein's stability, regulated activity and druggability. In light of this context, this article aims to review what we currently know about the structure, function and biogenesis of fungal Pma1.

Uncovering diffusive states of the yeast membrane protein, Pma1, and how labeling method can change diffusive behavior

We present and analyze video-microscopy-based single-particle-tracking measurements of the budding yeast (Saccharomyces cerevisiae) membrane protein, Pma1, fluorescently labeled either by direct fusion to the switchable fluorescent protein, mEos3.2, or by a novel, light-touch, labeling scheme, in which a 5 amino acid tag is directly fused to the C-terminus of Pma1, which then binds mEos3.2. The track diffusivity distributions of these two populations of single-particle tracks differ significantly, demonstrating that labeling method can be an important determinant of diffusive behavior. We also applied perturbation expectation maximization (pEMv2) (Koo and Mochrie in Phys Rev E 94(5):052412, 2016), which sorts trajectories into the statistically optimum number of diffusive states. For both TRAP-labeled Pma1 and Pma1-mEos3.2, pEMv2 sorts the tracks into two diffusive states: an essentially immobile state and a more mobile state. However, the mobile fraction of Pma1-mEos3.2 tracks is much smaller ([Formula: see text]) than the mobile fraction of TRAP-labeled Pma1 tracks ([Formula: see text]). In addition, the diffusivity of Pma1-mEos3.2's mobile state is several times smaller than the diffusivity of TRAP-labeled Pma1's mobile state. Thus, the two different labeling methods give rise to very different overall diffusive behaviors. To critically assess pEMv2's performance, we compare the diffusivity and covariance distributions of the experimental pEMv2-sorted populations to corresponding theoretical distributions, assuming that Pma1 displacements realize a Gaussian random process. The experiment-theory comparisons for both the TRAP-labeled Pma1 and Pma1-mEos3.2 reveal good agreement, bolstering the pEMv2 approach.

Structure of the hexameric fungal plasma membrane proton pump in its autoinhibited state

The fungal plasma membrane H⁺-ATPase Pma1 is a vital enzyme, generating a proton-motive force that drives the import of essential nutrients. Autoinhibited Pma1 hexamers in the plasma membrane of starving fungi are activated by glucose signaling and subsequent phosphorylation of the autoinhibitory domain. As related P-type adenosine triphosphatases (ATPases) are not known to oligomerize, the physiological relevance of Pma1 hexamers remained unknown. We have determined the structure of hexameric Pma1 from Neurospora crassa by electron cryo-microscopy at 3.3-Å resolution, elucidating the molecular basis for hexamer formation and autoinhibition and providing a basis for structure-based drug development. Coarse-grained molecular dynamics simulations in a lipid bilayer suggest lipid-mediated contacts between monomers and a substantial protein-induced membrane deformation that could act as a proton-attracting funnel.

Structure of the hexameric fungal plasma membrane proton pump in its autoinhibited state

The fungal plasma membrane H+-ATPase Pma1 is a vital enzyme, generating a proton-motive force that drives the import of essential nutrients. Autoinhibited Pma1 hexamers in the plasma membrane of starving fungi are activated by glucose signaling and subsequent phosphorylation of the autoinhibitory domain. As related P-type adenosine triphosphatases (ATPases) are not known to oligomerize, the physiological relevance of Pma1 hexamers remained unknown. We have determined the structure of hexameric Pma1 from Neurospora crassa by electron cryo-microscopy at 3.3-Å resolution, elucidating the molecular basis for hexamer formation and autoinhibition and providing a basis for structure-based drug development. Coarse-grained molecular dynamics simulations in a lipid bilayer suggest lipid-mediated contacts between monomers and a substantial protein-induced membrane deformation that could act as a proton-attracting funnel.

The Oligomeric State of the Plasma Membrane H⁺-ATPase from Kluyveromyces lactis

The plasma membrane H⁺-ATPase was purified from the yeast K. lactis. The oligomeric state of the H⁺-ATPase is not known. Size exclusion chromatography displayed two macromolecular assembly states (MASs) of different sizes for the solubilized enzyme. Blue native electrophoresis (BN-PAGE) showed the H⁺-ATPase hexamer in both MASs as the sole/main oligomeric state—in the aggregated and free state. The hexameric state was confirmed in dodecyl maltoside-treated plasma membranes by Western-Blot. Tetramers, dimers, and monomers were present in negligible amounts, thus depicting the oligomerization pathway with the dimer as the oligomerization unit. H⁺-ATPase kinetics was cooperative (n~1.9), and importantly, in both MASs significant differences were determined in intrinsic fluorescence intensity, nucleotide affinity and V_max; hence suggesting the large MAS as the activated state of the H⁺-ATPase. It is concluded that the quaternary structure of the H⁺-ATPase is the hexamer and that a relationship seems to exist between ATPase function and the aggregation state of the hexamer.

In vivo cross-linking supports a head-to-tail mechanism for regulation of the plant plasma membrane P-type H+-ATPase

In higher plants, a P-type proton-pumping ATPase generates the proton-motive force essential for the function of all other transporters and for proper growth and development. X-ray crystallographic studies of the plant plasma membrane proton pump have provided information on amino acids involved in ATP catalysis but provided no information on the structure of the C-terminal regulatory domain. Despite progress in elucidating enzymes involved in the signaling pathways that activate or inhibit this pump, the site of interaction of the C-terminal regulatory domain with the catalytic domains remains a mystery. Genetic studies have pointed to amino acids in various parts of the protein that may be involved, but direct chemical evidence for which ones are specifically interacting with the C terminus is lacking. In this study, we used in vivo cross-linking experiments with a photoreactive unnatural amino acid, p-benzoylphenylalanine, and tandem MS to obtain direct evidence that the C-terminal regulatory domain interacts with amino acids located within the N-terminal actuator domain. Our observations are consistent with a mechanism in which intermolecular, rather than intramolecular, interactions are involved. Our model invokes a "head-to-tail" organization of ATPase monomers in which the C-terminal domain of one ATPase molecule interacts with the actuator domain of another ATPase molecule. This model serves to explain why cross-linked peptides are found only in dimers and trimers, and it is consistent with prior studies suggesting that within the membrane the protein can be organized as homopolymers, including dimers, trimers, and hexamers.

Time-Resolved Investigation of Molecular Components Involved in the Induction of [Formula: see text] High Affinity Transport System in Maize Roots

The induction, i.e., the rapid increase of nitrate ([Formula: see text]) uptake following the exposure of roots to the anion, was studied integrating physiological and molecular levels in maize roots. Responses to [Formula: see text] treatment were characterized in terms of changes in [Formula: see text] uptake rate and plasma membrane (PM) H+-ATPase activity and related to transcriptional and protein profiles of NRT2, NRT3, and PM H+-ATPase gene families. The behavior of transcripts and proteins of ZmNRT2s and ZmNRT3s suggested that the regulation of the activity of inducible high-affinity transport system (iHATS) is mainly based on the transcriptional/translational modulation of the accessory protein ZmNRT3.1A. Furthermore, ZmNRT2.1 and ZmNRT3.1A appear to be associated in a ∼150 kDa oligomer. The expression trend during the induction of the 11 identified PM H+-ATPase transcripts indicates that those mainly involved in the response to [Formula: see text] treatment are ZmHA2 and ZmHA4. Yet, partial correlation between the gene expression, protein levels and enzyme activity suggests an involvement of post-transcriptional and post-translational mechanisms of regulation. A non-denaturing Deriphat-PAGE approach allowed demonstrating for the first time that PM H+-ATPase can occur in vivo as hexameric complex together with the already described monomeric and dimeric forms.

Active plasma membrane P-type H+-ATPase reconstituted into nanodiscs is a monomer

Plasma membrane H(+)-ATPases form a subfamily of P-type ATPases responsible for pumping protons out of cells and are essential for establishing and maintaining the crucial transmembrane proton gradient in plants and fungi. Here, we report the reconstitution of the Arabidopsis thaliana plasma membrane H(+)-ATPase isoform 2 into soluble nanoscale lipid bilayers, also termed nanodiscs. Based on native gel analysis and cross-linking studies, the pump inserts into nanodiscs as a functional monomer. Insertion of the H(+)-ATPase into nanodiscs has the potential to enable structural and functional characterization using techniques normally applicable only for soluble proteins.

Characterization of the plasma membrane H+-ATPase in the liverwort Marchantia polymorpha

The plasma membrane H(+)-ATPase generates an electrochemical gradient of H(+) across the plasma membrane that provides the driving force for solute transport and regulates pH homeostasis and membrane potential in plant cells. Recent studies have demonstrated that phosphorylation of the penultimate threonine in H(+)-ATPase and subsequent binding of a 14-3-3 protein is the major common activation mechanism for H(+)-ATPase in vascular plants. However, there is very little information on the plasma membrane H(+)-ATPase in nonvascular plant bryophytes. Here, we show that the liverwort Marchantia polymorpha, which is the most basal lineage of extant land plants, expresses both the penultimate threonine-containing H(+)-ATPase (pT H(+)-ATPase) and non-penultimate threonine-containing H(+)-ATPase (non-pT H(+)-ATPase) as in the green algae and that pT H(+)-ATPase is regulated by phosphorylation of its penultimate threonine. A search in the expressed sequence tag database of M. polymorpha revealed eight H(+)-ATPase genes, designated MpHA (for M. polymorpha H(+)-ATPase). Four isoforms are the pT H(+)-ATPase; the remaining isoforms are non-pT H(+)-ATPase. An apparent 95-kD protein was recognized by anti-H(+)-ATPase antibodies against an Arabidopsis (Arabidopsis thaliana) isoform and was phosphorylated on the penultimate threonine in response to the fungal toxin fusicoccin in thalli, indicating that the 95-kD protein contains pT H(+)-ATPase. Furthermore, we found that the pT H(+)-ATPase in thalli is phosphorylated in response to light, sucrose, and osmotic shock and that light-induced phosphorylation depends on photosynthesis. Our results define physiological signals for the regulation of pT H(+)-ATPase in the liverwort M. polymorpha, which is one of the earliest plants to acquire pT H(+)-ATPase.

The plant plasma membrane proton pump ATPase: a highly regulated P-type ATPase with multiple physiological roles

Around 40 P-type ATPases have been identified in Arabidopsis and rice, for which the genomes are known. None seems to exchange sodium and potassium, as does the animal Na(+)/K(+)-ATPase. Instead, plants, together with fungi, possess a proton pumping ATPase (H(+)-ATPase), which couples ATP hydrolysis to proton transport out of the cell, and so establishes an electrochemical gradient across the plasma membrane, which is dissipated by secondary transporters using protons in symport or antiport, as sodium is used in animal cells. Additional functions, such as stomata opening, cell growth, and intracellular pH homeostasis, have been proposed. Crystallographic data and homology modeling suggest that the H(+)-ATPase has a broadly similar structure to the other P-type ATPases but has an extended C-terminal region, which is involved in enzyme regulation. Phosphorylation of the penultimate residue, a Thr, and the subsequent binding of regulatory 14-3-3 proteins result in the formation of a dodecamer (six H(+)-ATPase and six 14-3-3 molecules) and enzyme activation. This type of regulation is unique to the P-type ATPase family. However, the recent identification of additional phosphorylated residues suggests further regulatory features.

Self-association of isolated large cytoplasmic domain of plasma membrane H+ -ATPase from Saccharomyces cerevisiae: role of the phosphorylation domain in a general dimeric model for P-ATPases

Large cytoplasmic domain (LCD) plasma membrane H+ -ATPase from S. cerevisiae was expressed as two fusion polypeptides in E. coli: a DNA sequence coding for Leu353-Ileu674 (LCDh), comprising both nucleotide (N) and phosphorylation (P) domains, and a DNA sequence coding for Leu353-Thr543 (LCDDeltah, lacking the C-terminus of P domain), were inserted in expression vectors pDEST-17, yielding the respective recombinant plasmids. Overexpressed fusion polypeptides were solubilized with 6 M urea and purified on affinity columns, and urea was removed by dialysis. Their predicted secondary structure contents were confirmed by CD spectra. In addition, both recombinant polypeptides exhibited high-affinity 2',3'-O-(2,4,6-trinitrophenyl)adenosine-5'-triphosphate (TNP-ATP) binding (Kd = 1.9 microM and 2.9 microM for LCDh and LCDDeltah, respectively), suggesting that they have native-like folding. The gel filtration profile (HPLC) of purified LCDh showed two main peaks, with molecular weights of 95 kDa and 39 kDa, compatible with dimeric and monomeric forms, respectively. However, a single elution peak was observed for purified LCDDeltah, with an estimated molecular weight of 29 kDa, as expected for a monomer. Together, these data suggest that LCDh exist in monomer-dimer equilibrium, and that the C-terminus of P domain is necessary for self-association. We propose that such association is due to interaction between vicinal P domains, which may be of functional relevance for H+ -ATPase in native membranes. We discuss a general dimeric model for P-ATPases with interacting P domains, based on published crystallography and cryo-electron microscopy evidence.

Multiple degradation pathways for misfolded mutants of the yeast plasma membrane ATPase, Pma1

To understand protein sorting and quality control in the secretory pathway, we have analyzed intracellular trafficking of the yeast plasma membrane ATPase, Pma1. Pma1 is ideal for such studies because it is a very abundant polytopic membrane protein, and its localization and activity at the plasma membrane are essential for cell viability and growth. We have tested whether the cytoplasmic amino- and carboxyl-terminal domains of Pma1 carry sorting information. As the sole copy of Pma1, mutants truncated at either NH2 or COOH termini are targeted at least partially to the plasma membrane and have catalytic activity to sustain cell viability. The mutants are also delivered to degradative pathways. Strikingly, NH2- and COOH-terminal Pma1 mutants are differentially recognized for degradation at distinct cellular locales. COOH-terminal mutants are recognized for destruction by endoplasmic reticulum-associated degradation. By contrast, NH2-terminal mutants escape detection by endoplasmic reticulum-associated degradation entirely, and undergo endocytosis for vacuolar degradation after apparently normal cell surface targeting. Both NH2- and COOH-terminal mutants are conformationally abnormal, as revealed by increased sensitivity to tryptic cleavage, but are able to assemble to form oligomers. We propose that different quality control mechanisms may assess discrete domains of Pma1 rather than a global conformational state.

Kinetic characterization of P-type membrane ATPase from Streptococcus mutans

The proton translocating membrane ATPase of oral streptococci has been implicated in cytoplasmatic pH regulation, acidurance and cariogenicity. Studies have confirmed that Streptococcus mutans is the most frequently detected species in dental caries. A P-type ATPase that can act together with F(1)F(o)-ATPase in S. mutans membrane has been recently described. The main objective of this work is to characterize the kinetic of ATP hydrolysis of this P-type ATPase. The optimum pH for ATP hydrolysis is around 6.0. The dependence of P-type ATPase activity on ATP concentration reveals high (K(0.5)=0.27 mM) and low (K(0.5)=3.31 mM) affinity sites for ATP, exhibiting positive cooperativity and a specific activity of about 74 U/mg. Equimolar concentrations of ATP and magnesium ions display a behavior similar to that described for ATP concentration in Mg(2+) saturating condition (high affinity site, K(0.5)=0.10 mM, and low affinity site, K(0.5)=2.12 mM), exhibiting positive cooperativity and a specific activity of about 68 U/mg. Sodium, potassium, ammonium, calcium and magnesium ions stimulate the enzyme, showing a single saturation curve, all exhibiting positive cooperativities, whereas inhibition of ATPase activity is observed for zinc ions and EDTA. The kinetic characteristics reveal that this ATPase belongs to type IIIA, like the ones found in yeast and plants.

A 100 kDa vanadate and lanzoprazole-sensitive ATPase from Streptococcus mutans membrane

The cariogenic potential of Streptococcus mutans is due to the production of organic acids derived from energy metabolism, which implies the need of mechanisms for the organism to tolerate this acidic environment. The F(1)F(o)-ATPase is generally considered as the main enzyme responsible for cytoplasmic proton extrusion, but mutations that resulted in a 50% reduction in F(1)F(o)-ATPase activity in S. mutans still allowed the micro-organism to grow and extrude acid, keeping the intracellular pH one pH unit above the extracellular ambient. This finding suggests the existence of other enzymatic (or cellular) mechanisms that keep the cytosolic pH neutral during micro-organism growth. This paper describes a membrane protein in S. mutans, with a molecular weight of 100 kDa, which exhibits ATPase activity inhibited by classic inhibitors of P-type ATPases (orthovanadate) and H(+),K(+)-ATPase (lanzoprazole), has an optimum pH comparable to other H(+)-ATPases and undergoes phosphorylation during the catalytic reaction, like that of H(+)-ATPases described in yeast and plant plasma membrane. Together, these results strongly suggest that the enzyme we describe here is a P-type H(+)-ATPase or H(+),ion-ATPase that can act in association with F(1)F(o)-ATPase during the growth of the S. mutans.

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.

Sphingoid base synthesis is required for oligomerization and cell surface stability of the yeast plasma membrane ATPase, Pma1

The plasma membrane H(+)-ATPase, Pma1, is an essential and long-lived integral membrane protein. Previous work has demonstrated that the Pma1-D378N mutant is a substrate for endoplasmic reticulum (ER)-associated degradation and causes a dominant negative effect on cell growth by preventing ER export of wild-type Pma1. We now show that Pma1-D378N is ubiquitylated, and it heterooligomerizes with wild-type Pma1, resulting in ubiquitylation and ER-associated degradation of wild-type Pma1. In temperature-sensitive lcb1-100 cells, defective in sphingoid base synthesis, Pma1 fails to oligomerize. At 30 degrees C, lcb1-100 is a suppressor of pma1-D378N because wild-type Pma1 fails to heterooligomerize with Pma1-D378N; wild-type Pma1 moves to the cell surface, indicating that oligomerization is not required for delivery to the plasma membrane. Even in the absence of Pma1-D378N, wild-type Pma1 is ubiquitylated and it undergoes internalization from the cell surface and vacuolar degradation at 30 degrees C in lcb1-100 cells. At 37 degrees C in lcb1-100 cells, a more severe defect occurs in sphingoid base synthesis, and targeting of newly synthesized Pma1 to the plasma membrane is impaired. These data indicate requirements for sphingolipids at three discrete stages: Pma1 oligomerization at the ER, targeting to the plasma membrane, and stability at the cell surface.

Ceramide biosynthesis is required for the formation of the oligomeric H+-ATPase Pma1p in the yeast endoplasmic reticulum

The yeast plasma membrane H(+)-ATPase Pma1p is one of the most abundant proteins to traverse the secretory pathway. Newly synthesized Pma1p exits the endoplasmic reticulum (ER) via COPII-coated vesicles bound for the Golgi. Unlike most secreted proteins, efficient incorporation of Pma1p into COPII vesicles requires the Sec24p homolog Lst1p, suggesting a unique role for Lst1p in ER export. Vesicles formed with mixed Sec24p-Lst1p coats are larger than those with Sec24p alone. Here, we examined the relationship between Pma1p biosynthesis and the requirement for this novel coat subunit. We show that Pma1p forms a large oligomeric complex of >1 MDa in the ER, which is packaged into COPII vesicles. Furthermore, oligomerization of Pma1p is linked to membrane lipid composition; Pma1p is rendered monomeric in cells depleted of ceramide, suggesting that association with lipid rafts may influence oligomerization. Surprisingly, monomeric Pma1p present in ceramide-deficient membranes can be exported from the ER in COPII vesicles in a reaction that is stimulated by Lst1p. We suggest that Lst1p directly conveys Pma1p into a COPII vesicle and that the larger size of mixed Sec24pLst1p COPII vesicles is not essential to the packaging of large oligomeric complexes.

PLANT PLASMA MEMBRANE H+-ATPases: Powerhouses for Nutrient Uptake

Most transport proteins in plant cells are energized by electrochemical gradients of protons across the plasma membrane. The formation of these gradients is due to the action of plasma membrane H+ pumps fuelled by ATP. The plasma membrane H+-ATPases share a membrane topography and general mechanism of action with other P-type ATPases, but differ in regulatory properties. Recent advances in the field include the identification of the complete H+-ATPase gene family in Arabidopsis, analysis of H+-ATPase function by the methods of reverse genetics, an improved understanding of the posttranslational regulation of pump activity by 14-3-3 proteins, novel insights into the H+ transport mechanism, and progress in structural biology. Furthermore, the elucidation of the three-dimensional structure of a related Ca2+ pump has implications for understanding of structure-function relationships for the plant plasma membrane H+-ATPase.

Thermal inactivation of the plasma membrane H+-ATPase from Kluyveromyces lactis. Protection by trehalose

The activity of the isolated plasma membrane H+-ATPase from Kluyveromyces lactis was measured during incubation at 35-45 degrees C and in the absence or in the presence of 0-0.6 M trehalose. As the temperature of incubation was raised from 35 to 45 degrees C, increasing enzyme inactivation rates were observed. Thermal inactivation kinetics of the H+-ATPase were biphasic exhibiting a first rapid phase and then a second slow phase. The transition from the native state occurred through a temperature-mediated increase in the inactivation rate constants of both phases. A model is proposed where the native H+-ATPase yields a partially active intermediary during the first phase of inactivation and then the intermediary is slowly converted into a totally inactive enzyme in the second phase. At each of these temperatures trehalose protected the enzymatic activity in a concentration dependent manner. Full protection was observed at 0.6 M trehalose in the range of 35-40 degrees C. Whereas, at 42 and 45 degrees C, the trehalose-mediated thermoprotection of the H+-ATPase was only partial. Trehalose stabilized the enzyme mainly by preventing the temperature dependent increase of the first and second inactivation rate constants.

The plant plasma membrane H(+)-ATPase: structure, function and regulation

The proton-pumping ATPase (H(+)-ATPase) of the plant plasma membrane generates the proton motive force across the plasma membrane that is necessary to activate most of the ion and metabolite transport. In recent years, important progress has been made concerning the identification and organization of H(+)-ATPase genes, their expression, and also the kinetics and regulation of individual H(+)-ATPase isoforms. At the gene level, it is now clear that H(+)-ATPase is encoded by a family of approximately 10 genes. Expression, monitored by in situ techniques, has revealed a specific distribution pattern for each gene; however, this seems to differ between species. In the near future, we can expect regulatory aspects of gene expression to be elucidated. Already the expression of individual plant H(+)-ATPases in yeast has shown them to have distinct enzymatic properties. It has also allowed regulatory aspects of this enzyme to be studied through random and site-directed mutagenesis, notably its carboxy-terminal region. Studies performed with both plant and yeast material have converged towards deciphering the way phosphorylation and binding of regulatory 14-3-3 proteins intervene in the modification of H(+)-ATPase activity. The production of high quantities of individual functional H(+)-ATPases in yeast constitutes an important step towards crystallization studies to derive structural information. Understanding the specific roles of H(+)-ATPase isoforms in whole plant physiology is another challenge that has been approached recently through the phenotypic analysis of the first transgenic plants in which the expression of single H(+)-ATPases has been up- or down-regulated. In conclusion, the progress made recently concerning the H(+)-ATPase family, at both the gene and protein level, has come to a point where we can now expect a more integrated investigation of the expression, function and regulation of individual H(+)-ATPases in the whole plant context.

Structure and function of the P-type ATPases

The P-type ATPases are integral membrane proteins that generate essential transmembrane ion gradients in virtually all living cells. The structures of two of these have recently been elucidated at a resolution of 8 A. When considered together with the large body of biochemical information that has accrued for these transporters and for enzymes in general, this new structural information is providing tantalizing insights regarding the molecular mechanism of active ion transport catalyzed by these proteins.

Trehalose-mediated protection of the plasma membrane H+-ATPase from Kluyveromyces lactis during freeze-drying and rehydration

During freeze-drying and rehydration, the activity of the H+-ATPase from the plasma membrane of Kluyveromyces lactis was preserved by increasing concentrations of carbohydrates. When the H+-ATPase was freeze-dried in the absence of carbohydrates the activity was lost. The protective efficiency of carbohydrates was as follows: trehalose > maltose > sucrose > glucose > galactose. Each carbohydrate exhibited the maximal protection at a concentration of 20 mg carbohydrate per milligram of protein or above. No structural changes of the rehydrated H+-ATPase were detected by intrinsic fluorescence measurements. Trehalose, at 20 mg/mg protein, protected the enzyme activity completely during freeze-drying and rehydration. Rehydration temperature was critical; at 20 degrees C or below, activity was fully retained, while at 30, 40, or 50 degrees C activity decreased in proportion with temperature. The trehalose-protected freeze-dried H+-ATPase was stored at different temperatures for up to 60 days. Storage at 4 degrees C resulted in retention of most of the enzymatic activity, while storage at 20 or 30 degrees C resulted in loss of activity. The protection of the H+-ATPase by trehalose suggests that this carbohydrate might protect other membrane enzymes from inactivation during handling.

Structure of the P-type ATPases

Electron cryocrystallography of precipitant-induced two-dimensional surface crystals of the neurospora plasma membrane H+ - ATPase and tubular crystals of the sarcoplasmic reticulum Ca(2+)-ATPase has recently yielded structure maps for these ion transporters at a resolution of about 8 A. The membrane-embedded regions of these closely related enzymes are similar, but the cytoplasmic regions appear to be significantly different.

Three-dimensional map of the plasma membrane H+-ATPase in the open conformation

The H+-ATPase from the plasma membrane of Neurospora crassa is an integral membrane protein of relative molecular mass 100K, which belongs to the P-type ATPase family that includes the plasma membrane Na+/K+-ATPase and the sarcoplasmic reticulum Ca2+-ATPase. The H+-ATPase pumps protons across the cell's plasma membrane using ATP as an energy source, generating a membrane potential in excess of 200mV. Despite the importance of P-type ATPases in controlling membrane potential and intracellular ion concentrations, little is known about the molecular mechanism they use for ion transport. This is largely due to the difficulty in growing well ordered crystals and the resulting lack of detail in the three-dimensional structure of these large membrane proteins. We have now obtained a three-dimensional map of the H+-ATPase by electron crystallography of two-dimensional crystals grown directly on electron microscope grids. At an in-plane resolution of 8 A, this map reveals ten membrane-spanning alpha-helices in the membrane domain, and four major cytoplasmic domains in the open conformation of the enzyme without bound ligands.

Mutations of pma-1, the gene encoding the plasma membrane H+-ATPase of Neurospora crassa, suppress inhibition of growth by concanamycin A, a specific inhibitor of vacuolar ATPases

Concanamycin A (CCA), a specific inhibitor of vacuolar ATPases, inhibited growth of Neurospora crassa in medium adjusted to pH 7 or above. Mutant strains were selected for growth on medium containing 1.0 microM CCA. Sixty-four (of 66) mutations mapped in the region of the pma1 locus, which encodes the plasma membrane H+-ATPase. Analysis of V-ATPase activity in isolated vacuolar membranes from the mutant strains showed wild-type activity and sensitivity to CCA. In contrast, plasma membrane H+-ATPase activity in isolated plasma membranes from the mutants was reduced as compared with wild-type, and in four strains the activity showed increased resistance to vanadate. The most interesting change in the plasma membrane H+-ATPase was in kinetic behavior. The wild-type enzyme showed sigmoid dependence on MgATP concentration with a Hill number of 2.0, while the seven mutants tested exhibited hyperbolic kinetics with a Hill number of 1.0. One interpretation of these data was that the enzyme had changed from a functional dimer to a functional monomer. Mutation of the plasma membrane H+-ATPase did not confer resistance by preventing uptake of CCA. In the presence of CCA both wild-type and mutant strains were unable to accumulate arginine, failed to concentrate chloroquine in acidic vesicles, and exhibited gross alterations in hyphal morphology, indicating that the CCA had entered the cells and inactivated the V-ATPase. Instead, we hypothesize that the mutations conferred resistance because the altered plasma membrane H+-ATPase could more efficiently rid the cell of toxic levels of Ca2+ or protons or other ions accumulated in the cytoplasm following inactivation of the V-ATPase by CCA.

Metabolic modulation of transport coupling ratio in yeast plasma membrane H(+)-ATPase

The plasma membrane proton pump (H(+)-ATPase) of yeast energizes solute uptake by secondary transporters and regulates cytoplasmic pH. The addition of glucose to yeast cells stimulates proton efflux mediated by the H(+)- ATPase. A > 50-fold increase in proton extrusion from yeast cells is observed in vivo, whereas the ATPase activity of purified plasma membranes is increased maximally 8-fold after glucose treatment (Serrano, R. (1983) FEBS Lett. 156, 11-14). The low capacity of yeast cells for proton extrusion in the absence of glucose can be explained by the finding that, in H(+)-ATPase isolated from glucose-starved cells, ATP hydrolysis is essentially uncoupled from proton pumping. The number of protons transported per ATP hydrolyzed is significantly increased after glucose activation. We suggest that intrinsic uncoupling is an important mechanism for regulation of pump activity.

Fourier transform infrared spectroscopy study of the secondary structure of the reconstituted Neurospora crassa plasma membrane H(+)-ATPase and of its membrane-associated proteolytic peptides

We reconstituted purified plasma membrane H(+)-ATPase from Neurospora crassa into soybean phospholipid vesicles (lipid/ATPase ratio of 5:1 w/w). The proteoliposomes contained an active ATPase, oriented inside-out. They were subjected to proteolysis by using Pronase, proteinase K, trypsin, and carboxypeptidase Y. Fourier transform infrared attenuated total reflection spectroscopy indicates that the amount of protein remaining after hydrolysis and elimination of the extramembrane domain of ATPase represents about 43% of the intact protein. The secondary structure of intact ATPase and of the membrane-associated domain of ATPase was determined by infrared spectroscopy. The membrane domain shows a typical alpha-helix and beta-sheet absorption. Polarized infrared spectroscopy reveals that the orientation of the helices is about perpendicular to the membrane. Amide hydrogen/deuterium exchange kinetics performed for the intact H(+)-ATPase and for the membrane-associated domain demonstrate that this part of ATPase shows less accessibility to the solvent than the entire protein but remains much more accessible to the solvent than bacteriorhodopsin membrane segments.

Reconstitution of the Neurospora crassa plasma membrane H(+)-adenosine triphosphatase

The purified H(+)-ATPase of the Neurospora crassa plasma membrane has been reconstituted by a gel filtration method into lipidic vesicles using sodium deoxycholate as the detergent. Reconstitution was performed for lipid/ATPase ratios ranging from 1000:1 to 5:1 (w/w). Whatever the lipid/ATPase ratio, the ATPase molecules completely associate with the lipid vesicles. The ATPase specific activity is identical for all proteoliposomes regardless of the lipid/ATPase ratio, but the H+ transport decreases at high protein/lipid ratios, suggesting that the proteoliposomes are more leaky to H+ as the amount of protein inserted into the lipidic membrane increases. Analysis of the fragments generated by trypsin proteolysis in the presence and in the absence of MgATP+ vanadate indicate that most of the reconstituted ATPase molecules are able to assume the transition state of the enzyme dephosphorylation reaction, and are therefore functional. The orientation (inside-out or rightside-out) of the ATPase molecules in the vesicles is independent of the lipid/ATPase ratio chosen for the reconstitution. For all the lipid/ATPase ratios tested, most of the ATPase molecules (> 99%) expose their cytoplasmic side to the outside of the reconstituted proteoliposomes. The size of the vesicles increases parallel to the ATPase amount. Although the H+ leakiness of our preparation at low lipid/protein ratios prevents proton pumping measurements, the reconstitution procedure described here has the main advantage on other procedures to allow the obtention of vesicles at high protein-to-lipid ratios, facilitating further structural characterization of the ATPase by biochemical and biophysical techniques. Therefore, the procedure described here could be of general interest in the field of membrane protein study.

Studies on the sedimentation behavior of the Neurospora crassa plasma membrane H(+)-ATPase synthesized in vitro and integrated into homologous microsomal membranes

RNA transcripts that encoded the Neurospora crassa plasma membrane H(+)-ATPase (pma+), a polytopic integral membrane protein, and the pma+344, a truncated pma+ with the amino terminal 344 amino acids, were translated in a N. crassa in vitro system. The microsomal membranes integrated products were insensitive to extraction by Na2CO3 (pH 11.5). The velocity sedimentation behavior of the in vitro synthesized pma+ were examined under various conditions. The pma+ migrated on linear sucrose gradients as aggregates which were heterogeneous in size, in the regions of 9-13 S; whereas, these values were reduced when Triton X-100 was presence in the gradients. The formation of these aggregates is interpreted to suggest a mechanism that maintains this polytopic integral membrane protein in a soluble form until it is targeted to the membranes. The sedimentation coefficient of the Triton X-100 solubilized microsomal membranes integrated pma+ corresponded roughly to a monomer of the pma+. Furthermore, a comparison of the trypsin cleavage patterns of the in vitro synthesized pma+ and of the microsomal membranes integrated pma+ suggest that they have different tertiary, or quaternary, structures. The latter did not give the characteristic trypsin cleavage patterns that have been observed for the native pma+ in the presence of its ligands MgATP and vanadate (Addison, R. and Scarborough, G.A. (1982) J. Biol. Chem. 257, 10421-10426). This was interpreted to suggest that the microsomal membranes integrated pma+ cannot interact with its substrate, suggesting that it is catalytically inactive.

Purification and characterization of the proton translocating plasma membrane ATPase of red beet storage tissue

Plasma membranes were prepared from red beet (Beta vulgaris L.) storage tissue by partition in an aqueous two-phase system. A highly active proton-translocating ATPase was purified from these membranes by lysophosphatidylcholine extraction and glycerol density gradient centrifugation. The ATPase activity was inhibited by vanadate or dicyclohexyl carbodiimide, but was insensitive to azide, nitrate and molybdate at concentrations which inhibit the F1ATPase, the tonoplast ATPase, and acid phosphatase. Inhibition by vanadate was consistent with a non-competitive mechanism, with Ki = 10 microM. The Km for Mg-ATP was about 1 mM, magnesium ions were required, and the activity was stimulated by KCl and by lysophosphatidylcholine. The optimal pH was 6.5. The molecular mass by gel filtration in the presence of 2 g/liter octyl glucoside was 600 kDa, while dodecyl sulfate gel electrophoresis gave a polypeptide molecular mass of 100 kDa. After blotting onto nitrocellulose, the purified enzyme did not bind concanavalin A, although a concanavalin A-binding peptide of the plasma membrane runs to nearly the same position on the gel and showed some tendency to co-purify with the ATPase. Phospholipid vesicles into which the purified ATPase had been incorporated by the freeze-thaw technique showed vanadate-sensitive, ATP-dependent proton uptake. When the ATPase was reconstituted into lipid membranes at high protein to lipid ratios and incubated with ATP, two-dimensionally crystalline arrays of protein molecules were formed.

A consensus structure for membrane transport

Combined information from biochemical and molecular biological experiments reveals a consistent structural rhythm that underlies the construction of all membrane carriers and perhaps all transport systems. Biochemical work shows that while some carrier proteins function as monomers, others operate as dimers. But despite this variation, all examples can be modelled as having a pair of membrane-embedded domains, each of which contains an array of (about) six transmembrane helical elements. This pattern is best documented among membrane carriers, where the minimal functional unit is known in a reasonable number of cases. Nevertheless, the same conclusion is likely to characterize other solute transporters. These unexpected correlations suggest that all membrane carriers, including those that take part in "energy coupling", have a uniform structural design on which is superimposed a variety of kinetic and biochemical mechanisms.

Transmembrane segments of the P-type cation-transporting ATPases. A comparative study

The transmembrane segments predicted for the Neurospora H-ATPase are laid out diagrammatically in Figure 10. Although the eight segments have arbitrarily been compressed into rectangles of the same size, they range in length from 20 residues (II) to 30 residues (IV and VI), so the corresponding helices must vary in length as well. Notable features of the model include the charged residues located just outside the plane of the membrane, with a clear excess of negative charges (5-, 1+) at the extracellular surface and a slight excess of positive charges (4+, 3-) at the cytoplasmic surface. There are also a conspicuous number of bulky residues (tryptophan, phenylalanine, and tyrosine) just inside the plane of the membrane. Within the bilayer, most of the helices are noticeably amphipathic, consistent with the expectation that at least some of them stack together to form a channel-like structure with a hydrophobic surface and a hydrophilic core. The charged residues predicted to lie within the membrane are listed in Table 2, which is a summary of data from eight of the P-type ATPases; the S. cerevisiae and S. pombe enzymes have not been included because they are nearly identical in this respect to the Neurospora enzyme. Interestingly, all of the ATPases have more membrane-embedded negative charges (5 to 8) than positive ones (0 to 4), a pattern that may be connected with their role as cation transporters. Certainly, other unrelated transport proteins have a rather different pattern of positive and negative charges: for example, the mammalian glucose transporter (1+, 2-), Na-glucose transporter (3+, 3-), and the E. coli lac permease (11+, 7-). The actual positioning of the negative charges in the P-type ATPases does not make it easy to single out the functionally important ones, however. The glutamyl residue in segment I is present in the fungal, plant, and Leishmania H-ATPases but not in the gastric H,K-ATPase. The same is true for the aspartate in segment II, except that it also appears in the muscle and brain Ca-ATPases. A glutamate is found at one end of segment III in the E. coli and fungal enzymes and at the other end in Arabidopsis; in segment IV, another glutamate appears in a well-conserved region in the Leishmania and mammalian enzymes but not in the bacterial, fungal, or plant ones.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for an essential histidine residue in the Neurospora crassa plasma membrane H+-ATPase

The Neurospora crassa plasma membrane H+-ATPase is rapidly inactivated in the presence of diethyl pyrocarbonate (DEP). The reaction is pseudo-first-order showing time- and concentration-dependent inactivation with a second-order rate constant of 385-420 M-1.min-1 at pH 6.9 and 25 degrees C. The difference spectrum of the native and modified enzyme has a maximum near 240 nm, characteristic of N-carbethoxyhistidine. No change in the absorbance of the inhibited ATPase at 278 nm or in the number of modifiable sulfhydryl groups is observed, indicating that the inhibition is not due to tyrosine or cysteine modification, and the inhibition is irreversible, ruling out serine residues. Furthermore, pretreatment of the ATPase with pyridoxal phosphate/NaBH4 under the conditions of the DEP treatment does not inhibit the ATPase and does not alter the DEP inhibition kinetics, indicating that the inactivation by DEP is not due to amino group modification. The pH dependence of the inactivation reaction indicates that the essential residue has a pKa near 7.5, and the activity lost as a result of H+-ATPase modification by DEP is partially recovered after hydroxylamine treatment at 4 degrees C. Taken together, these results strongly indicate that the inactivation of the H+-ATPase by DEP involves histidine modification. Analyses of the inhibition kinetics and the stoichiometry of modification indicate that among eight histidines modified per enzyme molecule, only one is essential for H+-ATPase activity. Finally, ADP protects against inactivation by DEP, indicating that the essential residue modified may be located at or near the nucleotide binding site.

Molecular properties of the fungal plasma-membrane [H+]-ATPase

The fungal plasma membrane contains a proton-translocating ATPase that is closely related, both structurally and functionally, to the [Na+, K+]-, [H+, K+]-, and [Ca2+]-ATPases of animal cells, the plasma-membrane [H+]-ATPase of higher plants, and several bacterial cation-transporting ATPases. This review summarizes currently available information on the molecular genetics, protein structure, and reaction cycle of the fungal enzyme. Recent efforts to dissect structure-function relationships are also discussed.