Substrate- and inhibitor-induced conformational changes in the yeast V-ATPase provide evidence for communication between the catalytic and proton-translocating sectors

The rice LATE ELONGATED HYPOCOTYL enhances salt tolerance by regulating Na+/K+ homeostasis and ABA signalling

The circadian clock plays multiple functions in the regulation of plant growth, development and response to various abiotic stress. Here, we showed that the core oscillator component late elongated hypocotyl (LHY) was involved in rice response to salt stress. The mutations of OsLHY gene led to reduced salt tolerance in rice. Transcriptomic analyses revealed that the OsLHY gene regulates the expression of genes related to ion homeostasis and the abscisic acid (ABA) signalling pathway, including genes encoded High-affinity K+ transporters (OsHKTs) and the stress-activated protein kinases (OsSAPKs). We demonstrated that OsLHY directly binds the promoters of OsHKT1;1, OsHKT1;4 and OsSAPK9 to regulate their expression. Moreover, the ossapk9 mutants exhibited salt tolerance under salt stress. Taken together, our findings revealed that OsLHY integrates ion homeostasis and the ABA pathway to regulate salt tolerance in rice, providing insights into our understanding of how the circadian clock controls rice response to salt stress.

A cell-based assay for CD63-containing extracellular vesicles

Extracellular vesicles (EVs) are thought to be important in cell-cell communication and have elicited extraordinary interest as potential biomarkers of disease. However, quantitative methods to enable elucidation of mechanisms underlying release are few. Here, we describe a cell-based assay for monitoring EV release using the EV-enriched tetraspanin CD63 fused to the small, ATP-independent reporter enzyme, Nanoluciferase. Release of CD63-containing EVs from stably expressing cell lines was monitored by comparing luciferase activity in culture media to that remaining in cells. HEK293, U2OS, U87 and SKMel28 cells released 0.3%-0.6% of total cellular CD63 in the form of EVs over 5 hrs, varying by cell line. To identify cellular machinery important for secretion of CD63-containing EVs, we performed a screen of biologically active chemicals in HEK293 cells. While a majority of compounds did not significantly affect EV release, treating cells with the plecomacrolides bafilomycin or concanamycin, known to inhibit the V-ATPase, dramatically increased EV release. Interestingly, alkalization of the endosomal lumen using weak bases had no effect, suggesting a pH-independent enhancement of EV release by V-ATPase inhibitors. The ability to quantify EVs in small samples will enable future detailed studies of release kinetics as well as further chemical and genetic screening to define pathways involved in EV secretion.

Energization of vacuolar transport in plant cells and its significance under stress

The plant vacuole is of prime importance in buffering environmental perturbations and in coping with abiotic stress caused by, for example, drought, salinity, cold, or UV. The large volume, the efficient integration in anterograde and retrograde vesicular trafficking, and the dynamic equipment with tonoplast transporters enable the vacuole to fulfill indispensible functions in cell biology, for example, transient and permanent storage, detoxification, recycling, pH and redox homeostasis, cell expansion, biotic defence, and cell death. This review first focuses on endomembrane dynamics and then summarizes the functions, assembly, and regulation of secretory and vacuolar proton pumps: (i) the vacuolar H(+)-ATPase (V-ATPase) which represents a multimeric complex of approximately 800 kDa, (ii) the vacuolar H(+)-pyrophosphatase, and (iii) the plasma membrane H(+)-ATPase. These primary proton pumps regulate the cytosolic pH and provide the driving force for secondary active transport. Carriers and ion channels modulate the proton motif force and catalyze uptake and vacuolar compartmentation of solutes and deposition of xenobiotics or secondary compounds such as flavonoids. ABC-type transporters directly energized by MgATP complement the transport portfolio that realizes the multiple functions in stress tolerance of plants.

Generation of chicken monoclonal antibodies against the a1, a2, and a3 subunit isoforms of vacuolar-type proton ATPase

The vacuolar-type proton pump ATPase (V-ATPase) plays several pivotal roles in the acidification of diverse intracellular compartments and the extracellular environment. The a subunit isoforms a1, a2, and a3, constituting the membrane-embedded section, are expressed in various tissues, and they are involved in the regulation of subcellular localization and activity of the holocomplex. Therefore, the characterization of their properties is indispensable for dissection of the physiological roles of the V-ATPase in highly differentiated cells. In this study, we report the production and characterization of chicken monoclonal antibodies (MAbs) against these mouse a1, a2 and a3 subunit isoforms. These MAbs are shown to be suitable for both immunoblotting and immunofluorescence analysis. The MAbs obtained in this study are useful in understanding the pathological basis of V-ATPase dysfunction.

The cellular energization state affects peripheral stalk stability of plant vacuolar H+-ATPase and impairs vacuolar acidification

The plant vacuolar H(+)-ATPase takes part in acidifying compartments of the endomembrane system including the secretory pathway and the vacuoles. The structural variability of the V-ATPase complex as well as its presence in different compartments and tissues involves multiple isoforms of V-ATPase subunits. Furthermore, a versatile regulation is essential to allow for organelle- and tissue-specific fine tuning. In this study, results from V-ATPase complex disassembly with a chaotropic reagent, immunodetection and in vivo fluorescence resonance energy transfer (FRET) analyses point to a regulatory mechanism in plants, which depends on energization and involves the stability of the peripheral stalks as well. Lowering of cellular ATP by feeding 2-deoxyglucose resulted in structural alterations within the V-ATPase, as monitored by changes in FRET efficiency between subunits VHA-E and VHA-C. Potassium iodide-mediated disassembly revealed a reduced stability of V-ATPase after 2-deoxyglucose treatment of the cells, but neither the complete V(1)-sector nor VHA-C was released from the membrane in response to 2-deoxyglucose treatment, precluding a reversible dissociation mechanism like in yeast. These data suggest the existence of a regulatory mechanism of plant V-ATPase by modification of the peripheral stator structure that is linked to the cellular energization state. This mechanism is distinct from reversible dissociation as reported for the yeast V-ATPase, but might represent an evolutionary precursor of reversible dissociation.

ATP induces conformational changes in the carboxyl-terminal region of ClC-5

ATP binding enhances the activity of ClC-5, the transporter mutated in Dent disease, a disease affecting the renal proximal tubule. Previously, the ATP binding site was revealed in x-ray crystal structures of the cytoplasmic region of this membrane protein. Disruption of this site by mutagenesis (Y617A-ClC-5) reduced the functional expression and ATP-dependent regulation of the full-length transporter in Xenopus oocytes. However, insight into the conformational changes underlying ATP-dependent regulation is lacking. Here, we show that ATP binding induces a change in protein conformation. Specifically, small angle x-ray scattering experiments indicate that ATP binding promotes a clamp-like closure of the isolated ClC-5 carboxyl-terminal region. Limited proteolysis studies show that ATP binding induces conformational compaction of the carboxyl-terminal region in the intact membrane protein as well. In the context of fibroblasts and proximal tubule epithelial cells, disruption of the ATP binding site in full-length ClC-5 (Y617A-ClC-5) led to a defect in processing and trafficking out of the endoplasmic reticulum. These latter findings account for the decrease in functional expression previously reported for this ATP-binding mutant and prompt future study of a model whereby conformational compaction caused by ATP binding promotes biosynthetic maturation.

Inhibition of osteoclast bone resorption by disrupting vacuolar H+-ATPase a3-B2 subunit interaction

Vacuolar H(+)-ATPases (V-ATPases) are highly expressed in ruffled borders of bone-resorbing osteoclasts, where they play a crucial role in skeletal remodeling. To discover protein-protein interactions with the a subunit in mammalian V-ATPases, a GAL4 activation domain fusion library was constructed from an in vitro osteoclast model, receptor activator of NF-κB ligand-differentiated RAW 264.7 cells. This library was screened with a bait construct consisting of a GAL4 binding domain fused to the N-terminal domain of V-ATPase a3 subunit (NTa3), the a subunit isoform that is highly expressed in osteoclasts (a1 and a2 are also expressed, to a lesser degree, whereas a4 is kidney-specific). One of the prey proteins identified was the V-ATPase B2 subunit, which is also highly expressed in osteoclasts (B1 is not expressed). Further characterization, using pulldown and solid-phase binding assays, revealed an interaction between NTa3 and the C-terminal domains of both B1 and B2 subunits. Dual B binding domains of equal affinity were observed in NTa, suggesting a possible model for interaction between these subunits in the V-ATPase complex. Furthermore, the a3-B2 interaction appeared to be moderately favored over a1, a2, and a4 interactions with B2, suggesting a mechanism for the specific subunit assembly of plasma membrane V-ATPase in osteoclasts. Solid-phase binding assays were subsequently used to screen a chemical library for inhibitors of the a3-B2 interaction. A small molecule benzohydrazide derivative was found to inhibit osteoclast resorption with an IC(50) of ∼1.2 μm on both synthetic hydroxyapatite surfaces and dentin slices, without significantly affecting RAW 264.7 cell viability or receptor activator of NF-κB ligand-mediated osteoclast differentiation. Further understanding of these interactions and inhibitors may contribute to the design of novel therapeutics for bone loss disorders, such as osteoporosis and rheumatoid arthritis.

Quantitative proteomics of the tonoplast reveals a role for glycolytic enzymes in salt tolerance

To examine the role of the tonoplast in plant salt tolerance and identify proteins involved in the regulation of transporters for vacuolar Na(+) sequestration, we exploited a targeted quantitative proteomics approach. Two-dimensional differential in-gel electrophoresis analysis of free flow zonal electrophoresis separated tonoplast fractions from control, and salt-treated Mesembryanthemum crystallinum plants revealed the membrane association of glycolytic enzymes aldolase and enolase, along with subunits of the vacuolar H(+)-ATPase V-ATPase. Protein blot analysis confirmed coordinated salt regulation of these proteins, and chaotrope treatment indicated a strong tonoplast association. Reciprocal coimmunoprecipitation studies revealed that the glycolytic enzymes interacted with the V-ATPase subunit B VHA-B, and aldolase was shown to stimulate V-ATPase activity in vitro by increasing the affinity for ATP. To investigate a physiological role for this association, the Arabidopsis thaliana cytoplasmic enolase mutant, los2, was characterized. These plants were salt sensitive, and there was a specific reduction in enolase abundance in the tonoplast from salt-treated plants. Moreover, tonoplast isolated from mutant plants showed an impaired ability for aldolase stimulation of V-ATPase hydrolytic activity. The association of glycolytic proteins with the tonoplast may not only channel ATP to the V-ATPase, but also directly upregulate H(+)-pump activity.

RANKL treatment releases the negative regulation of the poly(ADP-ribose) polymerase-1 on Tcirg1 gene expression during osteoclastogenesis

The Tcirg1 gene encodes the osteoclast-specific a3 isoform of the V-ATPase a subunit. Using the mouse osteoclastic model RAW264.7 cells, we studied Tcirg1 gene expression, and we identified PARP-1 as a transcriptional repressor negatively regulated by RANKL during osteoclastogenesis.

INTRODUCTION

The TCIRG1 gene encodes the a3 isoform of the V-ATPase a subunit, and mutations at this locus account for approximately 60% of infantile malignant osteopetrosis cases. Using RAW264.7 cells as an osteoclastic differentiation model, we undertook a transcriptional study of the mouse Tcirg1 gene focused on the 4-kb region upstream of the transcription starting point.

MATERIALS AND METHODS

The promoter activity of serial-deletion fragments of the Tcirg1 gene promoter was monitored throughout the RAW264.7 cell differentiation process. We next performed EMSA, UV cross-linking, affinity purification, mass spectrometry analysis, gel supershift, and siRNA transfection experiments to identify the factor(s) interacting with the promoter.

RESULTS

The -3946/+113 region of the mouse Tcirg1 gene displayed a high basal promoter activity, which was enhanced by RANKL treatment of RAW264.7 cells. Constructs deleted up to -1589 retained this response to RANKL. A deletion up to -1402 induced a 3-fold enhancement of the basal activity, whereas RANKL response was not affected. EMSA experiments led us to identify within the -1589/-1402 region, a 10-nucleotide sequence, which bound a nuclear protein present in nondifferentiated RAW264.7 cells. This interaction was lost using nuclear extracts derived from RANKL-treated cells. Affinity purification followed by mass spectrometry analysis and gel supershift assay allowed the identification of poly(ADP-ribose) polymerase-1 (PARP-1) as this transcriptional repressor, whereas Western blot experiments revealed the cleavage of the DNA-binding domain of PARP-1 on RANKL treatment. Finally, both PARP-1 depletion after siRNA transfection and RAW264.7 cell treatment by an inhibitor of PARP-1 activity induced an increase of a3 mRNA expression.

CONCLUSIONS

We provide evidence that the basal transcription activity of the Tcirg1 gene is negatively regulated by the binding of PARP-1 protein to its promoter region in mouse pre-osteoclast. On RANKL treatment, PARP-1 protein is cleaved and loses its repression effect, allowing an increase of Tcirg1 gene expression that is critical for osteoclast function.

Renal vacuolar H+-ATPase

Vacuolar H(+)-ATPases are ubiquitous multisubunit complexes mediating the ATP-dependent transport of protons. In addition to their role in acidifying the lumen of various intracellular organelles, vacuolar H(+)-ATPases fulfill special tasks in the kidney. Vacuolar H(+)-ATPases are expressed in the plasma membrane in the kidney almost along the entire length of the nephron with apical and/or basolateral localization patterns. In the proximal tubule, a high number of vacuolar H(+)-ATPases are also found in endosomes, which are acidified by the pump. In addition, vacuolar H(+)-ATPases contribute to proximal tubular bicarbonate reabsorption. The importance in final urinary acidification along the collecting system is highlighted by monogenic defects in two subunits (ATP6V0A4, ATP6V1B1) of the vacuolar H(+)-ATPase in patients with distal renal tubular acidosis. The activity of vacuolar H(+)-ATPases is tightly regulated by a variety of factors such as the acid-base or electrolyte status. This regulation is at least in part mediated by various hormones and protein-protein interactions between regulatory proteins and multiple subunits of the pump.

Topological analysis of a plant vacuolar Na+/H+ antiporter reveals a luminal C terminus that regulates antiporter cation selectivity

We conducted an analysis of the topology of AtNHX1, an Arabidopsis thaliana vacuolar Na+/H+ antiporter. Several hydrophilic regions of the antiporter were tagged with a hemagglutinin epitope, and protease protection assays were conducted to determine the membrane topology of the antiporter by using yeast as a heterologous expression system. The overall structure of AtNHX1 is distinct from the human Na+/H+ antiporter NHE1 or any known Na+/H+ antiporter. It is comprised of nine transmembrane domains and a hydrophilic C-terminal domain. Three hydrophobic regions do not appear to span the tonoplast membrane, yet appear to be membrane associated. Our results also indicate that, whereas the N terminus of AtNHX1 is facing the cytosol, almost the entire C-terminal hydrophilic region resides in the vacuolar lumen. Deletion of the hydrophilic C terminus resulted in a dramatic increase in the relative rate of Na+/H+ transport. The ratio of Na+/K+ transport was twice that of the unmodified AtNHX1. This altered ratio resulted from a relatively small decrease in K+/H+ transport with a large increase in Na+/H+ transport. The vacuolar localization of the C terminus of the AtNHX1, taken together with the regulation of the antiporter selectivity by its C terminus, demonstrates the existence of luminal vacuolar regulatory mechanisms of the antiporter activity.

The a3 isoform of the 100-kDa V-ATPase subunit is highly but differentially expressed in large (>or=10 nuclei) and small (<or= nuclei) osteoclasts

Osteoclasts dissolve bone through acidification of an extracellular compartment by means of a multimeric vacuolar type H+-ATPase (V-ATPase). In mammals, there are four isoforms of the 100-kDa V-ATPase "a" subunit. Mutations in the a3 isoform result in deficient bone resorption and osteopetrosis, suggesting that a3 has a unique function in osteoclasts. It is thus surprising that several studies show a basal level of a3 expression in most tissues. To address this issue, we have compared a3 expression in bone with expression in other tissues. RNA blots revealed that the a3 isoform was expressed highest in bone and confirmed its expression (in decreasing order) in liver, kidney, brain, lung, spleen, and muscle. In situ hybridization on bone tissue sections revealed that the a3 isoform was highly expressed in multinucleated osteoclasts but not in mononuclear stromal cells, whereas the a1 isoform was expressed in both cell types at about the same level. We also found that a3 expression was greater in osteoclasts with 10 or more nuclei as compared with osteoclasts with five or fewer nuclei. We hypothesize that these differences in a3 expression may be associated with previously demonstrated differences between large and small osteoclasts with reference to their resorptive activity.

Dynamics of the vacuolar H+-ATPase in the contractile vacuole complex and the endosomal pathway ofDictyosteliumcells

The vacuolar H+-ATPase (V-ATPase) is a multi-subunit enzyme that plays important roles in eukaryotic cells. In Dictyostelium, it is found primarily in membranes of the contractile vacuole complex, where it energizes fluid accumulation by this osmoregulatory organelle and also in membranes of endolysosomes, where it serves to acidify the endosomal lumen. In the present study, a fusion was created between vatM, the gene encoding the 100 kDa transmembrane subunit of the V-ATPase, and the gene encoding Green Fluorescent Protein (GFP). When expressed in Dictyostelium cells, this fusion protein, VatM-GFP, was correctly targeted to contractile vacuole and endolysosomal membranes and was competent to direct assembly of the V-ATPase enzyme complex. Protease treatment of isolated endosomes indicated that the GFP moiety, located on the C-terminus of VatM, was exposed to the cytoplasmic side of the endosomal membrane rather than to the lumenal side. VatM-GFP labeling of the contractile vacuole complex revealed clearly the dynamics of this pleiomorphic vesiculotubular organelle. VatM-GFP labeling of endosomes allowed direct visualization of the trafficking of vacuolar proton pumps in this pathway, which appeared to be entirely independent from the contractile vacuole membrane system. In cells whose endosomes were pre-labeled with TRITC-dextran and then fed yeast particles,VatM-GFP was delivered to newly formed yeast phagosomes with the same time course as TRITC-dextran, consistent with transfer via a direct fusion of endosomes with phagosomes. Several minutes were required before the intensity of the VatM-GFP labeling of new phagosomes reached the level observed in older phagosomes, suggesting that this fusion process was progressive and continuous. VatM-GFP was retrieved from the phagosome membrane prior to exocytosis of the indigestible remnants of the yeast particle. These data suggest that vacuolar proton pumps are recycled by fusion of advanced with newly formed endosomes.

Altered expression of the 100 kDa subunit of the Dictyosteliumvacuolar proton pump impairs enzyme assembly, endocytic function and cytosolic pH regulation

The vacuolar proton pump (V-ATPase) appears to be essential for viability of Dictyostelium cells. To investigate the function of VatM, the 100 kDa transmembrane V-ATPase subunit, we altered its level. By means of homologous recombination, the promoter for the chromosomal vatM gene was replaced with the promoter for the act6 gene, yielding the mutant strain VatMpr. The act6 promoter is much more active in cells growing axenically than on bacteria. Thus, transformants were selected under axenic growth conditions, then shifted to bacteria to determine the consequences of reduced vatM expression. When VatMpr cells were grown on bacteria,the level of the 100 kDa V-ATPase subunit dropped, cell growth slowed, and the A subunit, a component of the peripheral catalytic domain of the V-ATPase,became mislocalized. These defects were complemented by transformation of the mutant cells with a plasmid expressing vatM under the control of its own promoter. Although the principal locus of vacuolar proton pumps in Dictyostelium is membranes of the contractile vacuole system, mutant cells did not manifest osmoregulatory defects. However, bacterially grown VatMpr cells did exhibit substantially reduced rates of phagocytosis and a prolonged endosomal transit time. In addition, mutant cells manifested alterations in the dynamic regulation of cytosolic pH that are characteristic of normal cells grown in acid media, which suggested that the V-ATPase also plays a role in cytosolic pH regulation.

Three-dimensional map of a plant V-ATPase based on electron microscopy

V-ATPases pump protons into the interior of various subcellular compartments at the expense of ATP. Previous studies have shown that these pumps comprise a membrane-integrated, proton-translocating (V(0)), and a soluble catalytic (V(1)) subcomplex connected to one another by a thin stalk region. We present two three-dimensional maps derived from electron microscopic images of the complete V-ATPase complex from the plant Kalanchoë daigremontiana at a resolution of 2.2 nm. In the presence of a non-hydrolyzable ATP analogue, the details of the stalk region between V(0) and V(1) were revealed for the first time in their three-dimensional organization. A central stalk was surrounded by three peripheral stalks of different sizes and shapes. In the absence of the ATP analogue, the tilt of V(0) changed with respect to V(1), and the stalk region was less clearly defined, perhaps due to increased flexibility and partial detachment of some of the peripheral stalks. These structural changes corresponded to decreased stability of the complex and might be the initial step in a controlled disassembly.

Three-dimensional structure and subunit topology of the V(1) ATPase from Manduca sexta midgut

The three-dimensional structure of the Manduca sexta midgut V(1) ATPase has been determined at 3.2 nm resolution from electron micrographs of negatively stained specimens. The V(1) complex has a barrel-like structure 11 nm in height and 13.5 nm in diameter. It is hexagonal in the top view, whereas in the side view, the six large subunits A and B are interdigitated for most of their length (9 nm). The topology and importance of the individual subunits of the V(1) complex have been explored by protease digestion, resistance to chaotropic agents, MALDI-TOF mass spectrometry, and CuCl(2)-induced disulfide formation. Treatment of V(1) with trypsin or chaotropic iodide resulted in a rapid cleavage or release of subunit D from the enzyme, indicating that this subunit is exposed in the complex. Trypsin cleavage of V(1) decreased the ATPase activity with a time course that was in line with the cleavage of subunits B, C, G, and F. When CuCl(2) was added to V(1) in the presence of CaADP, the cross-linked products A-E-F and B-H were generated. In experiments where CuCl(2) was added after preincubation of CaATP, the cross-linked products E-F and E-G were formed. These changes in cross-linking of subunit E to near-neighbor subunits support the hypothesis that these are nucleotide-dependent conformational changes of the E subunit.

Properties of three isoforms of the 116-kDa subunit of vacuolar H+-ATPase from a single vertebrate species. Cloning, gene expression and protein characterization of functionally distinct isoforms in Gallus gallus

Vacuolar H+-ATPases (V-ATPases) are involved in a wide variety of essential cellular processes. An unresolved question is how the cell regulates the activity of these proton pumps and their targeting to distinct cellular compartments. There is growing evidence for the presence of subunit diversity amongst V-pumps, particularly regarding the 116-kDa subunit (called the a subunit). We have cloned and characterized three isoforms (a1, a2 and a3) of this subunit from chicken. The amino-acid sequences of these homologues are approximately 50% similar and their nucleotide differences indicate that they are products of distinct genes. The levels of mRNA expression of these isoforms was quantified by ribonuclease protection analysis. The a1 and a2 isoforms have a similar tissue distribution, with the highest level of mRNA expression in brain, an intermediate level in kidney and relatively low levels in liver and bone. In contrast, the highest level of expression of the a3 isoform is in bone and liver, with a moderate level in kidney, and the lowest level in brain. An antibody against the a1 isoform reacted with a 116 kDa protein in a brain V-ATPase preparation that was not detected in bone or liver V-ATPase preparations, whereas an antibody against the a3 isoform reacted with a 116-kDa peptide in bone and liver, but not brain V-ATPases preparations. The bone and brain V-ATPases showed differential sensitivity to the inhibitors bafilomycin and (2Z,4E)-5-(5,6-dichloro-2-indolyl)-2-methoxy-N-[4-(2, 2,6,6-tetramethyl)piperidinyl]-2,4-pentadienamide. Thus, this work demonstrates the presence of structurally and functionally distinct V-ATPases in a single vertebrate species.

Evidence that the NH2 terminus of vph1p, an integral subunit of the V0 sector of the yeast V-ATPase, interacts directly with the Vma1p and Vma13p subunits of the V1 sector

The vacuolar-type H(+)-ATPase (V-ATPase) is composed of a peripherally bound (V(1)) and a membrane-associated (V(0)) complex. V(1) ATP hydrolysis is thought to rotate a central stalk, which in turn, is hypothesized to drive V(0) proton translocation. Transduction of torque exerted by the rotating stalk on V(0) requires a fixed structural link (stator) between the complexes to prevent energy loss through futile rotation of V(1) relative to V(0); this work sought to identify stator components. The 95-kDa V-ATPase subunit, Vph1p, has a cytosolic NH(2) terminus (Nt-Vph1p) and a membrane-associated COOH terminus. Two-hybrid assays demonstrated that Nt-Vph1p interacts with the catalytic V(1) subunit, Vma1p. Co-immunoprecipitation of Vma1p with Nt-Vph1p confirmed the interaction. Expression of Nt-Vph1p in a Deltavph1 mutant was necessary to recruit Vma13p to V(1). Vma13p bound to Nt-Vph1p in vitro demonstrating direct interaction. Limited trypsin digests cleaves both Nt-Vph1p and Vma13p. The same tryptic treatment results in a loss of proton translocation while not reducing bafilomycin A(1)-sensitive ATP hydrolysis. Trypsin cleaved Vph1p at arginine 53. Elimination of the tryptic cleavage site by substitution of arginine 53 to serine partially protected vacuolar acidification from trypsin digestion. These results suggest that Vph1p may function as a component of a fixed structural link, or stator, coupling V(1) ATP hydrolysis to V(0) proton translocation.

Regulation of V-ATPases by reversible disassembly

V-ATPases consist of a complex of peripheral subunits containing catalytic sites for ATP hydrolysis, the V(1) sector, attached to several membrane subunits containing a proton pore, the V(0) sector. ATP-driven proton transport requires structural and functional coupling of the two sectors, but in vivo, the interaction between the V(1) and V(0) sectors is dynamic and is regulated by extracellular conditions. Dynamic instability appears to be a general characteristic of V-ATPases and, in yeast cells, the assembly state of V-ATPases is governed by glucose availability. The structural and functional implications of reversible disassembly of V-ATPases are discussed.