Subunit H of the vacuolar (H+) ATPase inhibits ATP hydrolysis by the free V1 domain by interaction with the rotary subunit F

Identification of Autophagy-Related Candidate Genes in the Early Diagnosis of Alzheimer's Disease and Exploration of Potential Molecular Mechanisms

This study aimed to identify autophagy-related candidate genes for the early diagnosis of Alzheimer's disease (AD) and elucidate their potential molecular mechanisms. Differentially expressed genes (DEGs) and phenotype-associated significant module genes were obtained using the "limma" package and weighted gene co-expression network analysis (WGCNA) based on hippocampal tissue datasets from AD patients and control samples. The intersection between the list of autophagy-related genes (ATGs), DEGs, and module genes was further investigated to obtain AD-autophagy-related differential expression genes (ATDEGs). Subsequently, the least absolute shrinkage and selection operator (LASSO) algorithm was utilized to identify hub genes, and a second intersection was performed with important module genes from the protein-protein interaction (PPI) network to obtain co-hub genes. Finally, a diagnostic model was constructed by receiver operating characteristic (ROC) analysis to determine the candidate genes with high diagnostic efficacy in the external validation set. Moreover, immune infiltration analysis was performed on AD patient brain tissues and explore the correlation between candidate genes and immune cells. We further analyzed the expression level of candidate genes in the SH-SY5Y cells with Aβ25-35 (25 µM). Among the 17 identified AD-ATDEGs, ATP6V1E1 stood out with area under the curve (AUC) values of 0.869, 0.817, and 0.714 in the external validation set, underscoring its high diagnostic efficacy in both hippocampal and peripheral blood contexts for AD patients. Meanwhile, ATP6V1E1 expression was positively correlated with effector memory CD4 + T cells, while negatively correlated with natural killer T cells and activated CD4 + T cells. Results from quantitative PCR (qPCR) and immunofluorescence assays indicated a reduction in ATP6V1E1 expression, aligning with our database analysis findings. In summary, ATP6V1E1 as a candidate gene provides a new perspective for the early identification and pathogenesis of AD.

TFEB-vacuolar ATPase signaling regulates lysosomal function and microglial activation in tauopathy

Transcription factor EB (TFEB) mediates gene expression through binding to the coordinated lysosome expression and regulation (CLEAR) sequence. TFEB targets include subunits of the vacuolar ATPase (v-ATPase), which are essential for lysosome acidification. Single-nucleus RNA sequencing of wild-type and PS19 (Tau) transgenic mice expressing the P301S mutant tau identified three unique microglia subclusters in Tau mice that were associated with heightened lysosome and immune pathway genes. To explore the lysosome-immune relationship, we specifically disrupted the TFEB-v-ATPase signaling by creating a knock-in mouse line in which the CLEAR sequence of one of the v-ATPase subunits, Atp6v1h, was mutated. CLEAR mutant exhibited a muted response to TFEB, resulting in impaired lysosomal acidification and activity. Crossing the CLEAR mutant with Tau mice led to higher tau pathology but diminished microglia response. These microglia were enriched in a subcluster low in mTOR and HIF-1 pathways and were locked in a homeostatic state. Our studies demonstrate a physiological function of TFEB-v-ATPase signaling in maintaining lysosomal homeostasis and a critical role of the lysosome in mounting a microglia and immune response in tauopathy and Alzheimer's disease.

Integrated transcriptomics reveals the brain and blood biomarkers in Alzheimer's disease

BACKGROUND

The systematic molecular associations between the peripheral blood cells and brain in Alzheimer's disease (AD) remains unclear, which hinders our understanding of AD pathological mechanisms and the exploration of new diagnostic biomarkers.

METHODS

Here, we performed an integrated analysis of the brain and peripheral blood cells transcriptomics to establish peripheral biomarkers of AD. By employing multiple statistical analyses plus machine learning, we identified and validated multiple regulated central and peripheral network in patients with AD.

RESULTS

By bioinformatics analysis, a total of 243 genes were differentially expressed in the central and peripheral systems, mainly enriched in three modules: immune response, glucose metabolism and lysosome. In addition, lysosome related gene ATP6V1E1 and immune response related genes (IL2RG, OSM, EVI2B TNFRSF1A, CXCR4, STAT5A) were significantly correlated with Aβ or Tau pathology. Finally, receiver operating characteristic (ROC) analysis revealed that ATP6V1E1 showed high-diagnostic potential for AD.

CONCLUSION

Taken together, our data identified the main pathological pathways in AD progression, particularly the systemic dysregulation of the immune response, and provided peripheral biomarkers for AD diagnosis.

Coordinated conformational changes in the V1 complex during V-ATPase reversible dissociation

Vacuolar-type ATPases (V-ATPases) are rotary enzymes that acidify intracellular compartments in eukaryotic cells. These multi-subunit complexes consist of a cytoplasmic V₁ region that hydrolyzes ATP and a membrane-embedded V_O region that transports protons. V-ATPase activity is regulated by reversible dissociation of the two regions, with the isolated V₁ and V_O complexes becoming autoinhibited on disassembly and subunit C subsequently detaching from V₁. In yeast, assembly of the V₁ and V_O regions is mediated by the regulator of the ATPase of vacuoles and endosomes (RAVE) complex through an unknown mechanism. We used cryogenic-electron microscopy of yeast V-ATPase to determine structures of the intact enzyme, the dissociated but complete V₁ complex and the V₁ complex lacking subunit C. On separation, V₁ undergoes a dramatic conformational rearrangement, with its rotational state becoming incompatible for reassembly with V_O. Loss of subunit C allows V₁ to match the rotational state of V_O, suggesting how RAVE could reassemble V₁ and V_O by recruiting subunit C.

Molecular characterization and expression profiles of six genes involved in vitellogenic deposition and hydrolysis of Chinese sturgeon (Acipenser sinensis) suggesting their transcriptional regulation on ovarian development

Ovary development of Chinese sturgeon (Acipenser sinensis) in controlled breeding has been reported to respond to dietary lipid levels. However, the corresponding molecular regulatory mechanism about ovary development of Chinese sturgeon is still unclear. To elucidate the molecular mechanism of vitellogenic deposition and hydrolysis, six key genes, namely, vtgr (vitellogenin receptor), atp6v1c1 (Vacuolar H⁺-ATPase subunit c1), atp6v1h (Vacuolar H⁺-ATPase subunit h), ctsb (cathepsin B), ctsd (cathepsin D) and ctsl (cathepsin L) involved in vitellogenic deposition and hydrolysis of Chinese sturgeon were cloned and characterized, and their spatio-temporal mRNA expression profiles as well as transcriptional responses to dietary lipid level were investigated. The full-length cDNA sequences of these six genes showed similar domain structure to their respective orthologous genes from other vertebrates. Tissue-specific expression patterns of these genes were observed in ovary, liver, muscle, spleen, brain, gill, intestine, heart, stomach and kidney. Ovarian expression level of vtgr was the highest in stage II, and ctsl expression was the highest in stage IV, while the mRNA expressions of other 4 genes were the highest in stage III. The increase of dietary lipid level promoted ovary development and elevated the expressions of vtgr, atp6v1c1, atp6v1h, ctsb and ctsd in the ovary. The results of the present study indicated that these genes are crucial for vitellogenic deposition, and provided a preliminary understanding on the molecular regulation of vitellogenic deposition and hydrolysis during ovary development of Chinese sturgeon.

Lysosome as a Central Hub for Rewiring PH Homeostasis in Tumors

Cancer cells generate large quantities of cytoplasmic protons as byproducts of aberrantly activated aerobic glycolysis and lactate fermentation. To avoid potentially detrimental acidification of the intracellular milieu, cancer cells activate multiple acid-removal pathways that promote cytosolic alkalization and extracellular acidification. Accumulating evidence suggests that in addition to the well-characterized ion pumps and exchangers in the plasma membrane, cancer cell lysosomes are also reprogrammed for this purpose. On the one hand, the increased expression and activity of the vacuolar-type H⁺-ATPase (V-ATPase) on the lysosomal limiting membrane combined with the larger volume of the lysosomal compartment increases the lysosomal proton storage capacity substantially. On the other hand, enhanced lysosome exocytosis enables the efficient release of lysosomal protons to the extracellular space. Together, these two steps dynamically drive proton flow from the cytosol to extracellular space. In this perspective, we provide mechanistic insight into how lysosomes contribute to the rewiring of pH homeostasis in cancer cells.

Regulation and function of V-ATPases in physiology and disease

The vacuolar H⁺-ATPases (V-ATPases) are essential, ATP-dependent proton pumps present in a variety of eukaryotic cellular membranes. Intracellularly, V-ATPase-dependent acidification functions in such processes as membrane traffic, protein degradation, autophagy and the coupled transport of small molecules. V-ATPases at the plasma membrane of certain specialized cells function in such processes as bone resorption, sperm maturation and urinary acidification. V-ATPases also function in disease processes such as pathogen entry and cancer cell invasiveness, while defects in V-ATPase genes are associated with disorders such as osteopetrosis, renal tubular acidosis and neurodegenerative diseases. This review highlights recent advances in our understanding of V-ATPase structure, mechanism, function and regulation, with an emphasis on the signaling pathways controlling V-ATPase assembly in mammalian cells. The role of V-ATPases in cancer and other human pathologies, and the prospects for therapeutic intervention, are also discussed.

Systemic RNAi of V-ATPase subunit B causes molting defect and developmental abnormalities in Periplaneta fuliginosa

The vacuolar (H⁺ )-ATPases (V-ATPases) are ATP-driven proton pumps with multiple functions in many organisms. In this study, we performed structural and functional analysis of vha55 gene that encodes V-ATPase subunit B in the smokybrown cockroach Periplaneta fuliginosa (Blattodea). We observed a high homology score of the deduced amino acid sequences between 10 species in seven orders. RNAi of the vha55 gene in P. fuliginosa caused nymphal/nymphal molting defects with incomplete shedding of old cuticles, growth inhibition, as well as bent and wrinkled cuticles of thoraxes and abdominal segments. Since growth inhibition caused by vha55 RNAi did not interfere in the commencement of cockroach molting, molting timing and body growth might be controlled by independent mechanism. Our study suggested V-ATPases might be a good candidate molecule for evolutionary and developmental studies of insect molting.

Specific V-ATPase expression sub-classifies IDHwt lower-grade gliomas and impacts glioma growth in vivo

Background

Cancer cells use specific V-ATPase subunits to activate oncogenic pathways. Therefore, we investigated V-ATPase deregulation in aggressive gliomas and associated signaling.

Methods

V-ATPase genes expression and associated pathways were analyzed in different series of glioma available from public databases, as well as in patients' cohort. Activation of pathways was analyzed at gene and protein expression levels. A genetic model of glioma in Drosophila melanogaster and mice with GBM patients-derived orthotopic xenografts were used as in vivo models of disease.

Findings

GBM and recurrent gliomas display a specific V-ATPase signature. Such signature resolves the heterogeneous class of IDH-wild type lower-grade gliomas, identifying the patients with worse prognosis independently from clinical and molecular features (p = 0·03, by Cox proportional-hazards model). In vivo, V-ATPase subunits deregulation significantly impacts tumor growth and proliferation. At the molecular level, GBM-like V-ATPase expression correlates with upregulation of Homeobox genes.

Interpretation

Our data identify a V-ATPase signature that accompanies glioma aggressiveness and suggest new entry points for glioma stratification and follow-up.

Fund

This work was supported by Fondazione Cariplo (2014–1148 to VV), Fondazione IRCCS Ca' Granda, and Fondazione INGM Grant in Molecular Medicine 2014 (to VV).

The Vacuolar ATPase - A Nano-scale Motor That Drives Cell Biology

The vacuolar H⁺-ATPase (V-ATPase) is a ~1 MDa membrane protein complex that couples the hydrolysis of cytosolic ATP to the transmembrane movement of protons. In essentially all eukaryotic cells, this acid pumping function plays critical roles in the acidification of endosomal/lysosomal compartments and hence in transport, recycling and degradative pathways. It is also important in acid extrusion across the plasma membrane of some cells, contributing to homeostatic control of cytoplasmic pH and maintenance of appropriate extracellular acidity. The complex, assembled from up to 30 individual polypeptides, operates as a molecular motor with rotary mechanics. Historically, structural inferences about the eukaryotic V-ATPase and its subunits have been made by comparison to the structures of bacterial homologues. However, more recently, we have developed a much better understanding of the complete structure of the eukaryotic complex, in particular through advances in cryo-electron microscopy. This chapter explores these recent developments, and examines what they now reveal about the catalytic mechanism of this essential proton pump and how its activity might be regulated in response to cellular signals.

Clathrin coat controls synaptic vesicle acidification by blocking vacuolar ATPase activity

Newly-formed synaptic vesicles (SVs) are rapidly acidified by vacuolar adenosine triphosphatases (vATPases), generating a proton electrochemical gradient that drives neurotransmitter loading. Clathrin-mediated endocytosis is needed for the formation of new SVs, yet it is unclear when endocytosed vesicles acidify and refill at the synapse. Here, we isolated clathrin-coated vesicles (CCVs) from mouse brain to measure their acidification directly at the single vesicle level. We observed that the ATP-induced acidification of CCVs was strikingly reduced in comparison to SVs. Remarkably, when the coat was removed from CCVs, uncoated vesicles regained ATP-dependent acidification, demonstrating that CCVs contain the functional vATPase, yet its function is inhibited by the clathrin coat. Considering the known structures of the vATPase and clathrin coat, we propose a model in which the formation of the coat surrounds the vATPase and blocks its activity. Such inhibition is likely fundamental for the proper timing of SV refilling.

Analysis of two transcript isoforms of vacuolar ATPase subunit H in mouse and zebrafish

ATP6V1H encodes the subunit H of vacuolar ATPase (V-ATPase) and has been recently proved to regulate osteoclast function. The alternative splicing of ATP6V1H gene results in two isoforms, and it is not clear whether and how the two isoforms function differently. In this report, we used bioinformatics methods to compare the differences of two isoforms in different species. The distributions and amounts of two isoforms were analyzed in eleven kinds of mouse tissues and mouse osteoclasts using RT-PCR, Q-PCR, western blot and immunohistochemical staining methods, respectively. In order to observe the in vivo biological differences of two isoforms during development, the zebrafish mRNA of two wild type atp6v1h transcripts as well as their mutant forms were also injected into zebrafish embryos, respectively. Bioinformatic analysis revealed that two isoforms were quite different in many ways, especially in protein size, internal space, phosphorylation state and H-bond binding. The amounts of two transcripts and the ratio of long and short transcript varied a lot from tissue to tissue or cell to cell, and osteoclasts were the cells only expressing long isoform among the tissues or cells we detected. The in vivo selective expression of two subunit H splice variants showed their different effects on the craniofacial development of zebrafish. The short isoform reduced the size of zebrafish head and did not play a complete function compared with the long isoform. We propose that long isoform of subunit H is necessary for the normal craniofacial bone development and the lack of short transcript might be necessary for the normal osteoclastic function.

Rotating with the brakes on and other unresolved features of the vacuolar ATPase

The rotary ATPase family comprises the ATP synthase (F-ATPase), vacuolar ATPase (V-ATPase) and archaeal ATPase (A-ATPase). These either predominantly utilize a proton gradient for ATP synthesis or use ATP to produce a proton gradient, driving secondary transport and acidifying organelles. With advances in EM has come a significant increase in our understanding of the rotary ATPase family. Following the sub nm resolution reconstructions of both the F- and V-ATPases, the secondary structure organization of the elusive subunit a has now been resolved, revealing a novel helical arrangement. Despite these significant developments in our understanding of the rotary ATPases, there are still a number of unresolved questions about the mechanism, regulation and overall architecture, which this mini-review aims to highlight and discuss.

ATP6V1H Deficiency Impairs Bone Development through Activation of MMP9 and MMP13

ATP6V1H is a component of a large protein complex with vacuolar ATPase (V-ATPase) activity. We identified two generations of individuals in which short stature and osteoporosis co-segregated with a mutation in ATP6V1H. Since V-ATPases are highly conserved between human and zebrafish, we generated loss-of-function mutants in atp6v1h in zebrafish through CRISPR/Cas9-mediated gene knockout. Homozygous mutant atp6v1h zebrafish exhibited a severe reduction in the number of mature calcified bone cells and a dramatic increase in the expression of mmp9 and mmp13. Heterozygous adults showed curved vertebra that lack calcified centrum structure and reduced bone mass and density. Treatment of mutant embryos with small molecule inhibitors of MMP9 and MMP13 significantly restored bone mass in the atp6v1h mutants. These studies have uncovered a new, ATP6V1H-mediated pathway that regulates bone formation, and defines a new mechanism of disease that leads to bone loss. We propose that MMP9/MMP13 could be therapeutic targets for patients with this rare genetic disease.

Osteoporosis, a major health problem worldwide, is characterized by low bone mineral density (BMD) and a propensity to fracture. Genetic factors are clearly the major determinants of BMD, but their identification and contribution to osteoporosis risk have been difficult to assess in humans. Genome-wide association studies (GWAS) have identified numerous sequence variants that influence BMD. The loci identified to date, however, account for only a small fraction of the total variation in BMD. Through analysis of a pedigree with an undiagnosed disease in which affected members have markedly low bone mass, we identify a novel and critical bone formation pathway, mediated through the gene ATP6V1H. Zebrafish lacking apt6vh1 demonstrate loss of bone mass, and exhibit increased mmp9 and mmp13 levels; inhibition of mmp9 and mmp13 led to the rescue of bone density defects. Here we show that happloinsufficiency of ATP6V1H is associated with osteoporosis in both humans and zebrafish. This study exemplifies the value of studying rare diseases to understand prevalent ones.

Crystal structure of yeast V1-ATPase in the autoinhibited state

Vacuolar ATPases (V-ATPases) are essential proton pumps that acidify the lumen of subcellular organelles in all eukaryotic cells and the extracellular space in some tissues. V-ATPase activity is regulated by a unique mechanism referred to as reversible disassembly, wherein the soluble catalytic sector, V1, is released from the membrane and its MgATPase activity silenced. The crystal structure of yeast V1 presented here shows that activity silencing involves a large conformational change of subunit H, with its C-terminal domain rotating ~150° from a position near the membrane in holo V-ATPase to a position at the bottom of V1 near an open catalytic site. Together with biochemical data, the structure supports a mechanistic model wherein subunit H inhibits ATPase activity by stabilizing an open catalytic site that results in tight binding of inhibitory ADP at another site.

Regulation of Vacuolar H+-ATPase (V-ATPase) Reassembly by Glycolysis Flow in 6-Phosphofructo-1-kinase (PFK-1)-deficient Yeast Cells

Yeast 6-phosphofructo-1-kinase (PFK-1) has two subunits, Pfk1p and Pfk2p. Deletion of Pfk2p alters glucose-dependent V-ATPase reassembly and vacuolar acidification (Chan, C. Y., and Parra, K. J. (2014) Yeast phosphofructokinase-1 subunit Pfk2p is necessary for pH homeostasis and glucose-dependent vacuolar ATPase reassembly. J. Biol. Chem. 289, 19448-19457). This study capitalized on the mechanisms suppressing vacuolar H(+)-ATPase (V-ATPase) in pfk2Δ to gain new knowledge of the mechanisms underlying glucose-dependent V-ATPase regulation. Because V-ATPase is fully assembled in pfk2Δ, and glycolysis partially suppressed at steady state, we manipulated glycolysis and assessed its direct involvement on V-ATPase function. At steady state, the ratio of proton transport to ATP hydrolysis increased 24% after increasing the glucose concentration from 2% to 4% to enhance the glycolysis flow in pfk2Δ. Tighter coupling restored vacuolar pH when glucose was abundant and glycolysis operated below capacity. After readdition of glucose to glucose-deprived cells, glucose-dependent V1Vo reassembly was proportional to the glycolysis flow. Readdition of 2% glucose to pfk2Δ cells, which restored 62% of ethanol concentration, led to equivalent 60% V1Vo reassembly levels. Steady-state level of assembly (100% reassembly) was reached at 4% glucose when glycolysis reached a threshold in pfk2Δ (≥40% the wild-type flow). At 4% glucose, the level of Pfk1p co-immunoprecipitated with V-ATPase decreased 58% in pfk2Δ, suggesting that Pfk1p binding to V-ATPase may be inhibitory in the mutant. We concluded that V-ATPase activity at steady state and V-ATPase reassembly after readdition of glucose to glucose-deprived cells are controlled by the glycolysis flow. We propose a new mechanism by which glucose regulates V-ATPase catalytic activity that occurs at steady state without changing V1Vo assembly.

Recent Insights into the Structure, Regulation, and Function of the V-ATPases

The vacuolar (H(+))-ATPases (V-ATPases) are ATP-dependent proton pumps that acidify intracellular compartments and are also present at the plasma membrane. They function in such processes as membrane traffic, protein degradation, virus and toxin entry, bone resorption, pH homeostasis, and tumor cell invasion. V-ATPases are large multisubunit complexes, composed of an ATP-hydrolytic domain (V1) and a proton translocation domain (V0), and operate by a rotary mechanism. This review focuses on recent insights into their structure and mechanism, the mechanisms that regulate V-ATPase activity (particularly regulated assembly and trafficking), and the role of V-ATPases in processes such as cell signaling and cancer. These developments have highlighted the potential of V-ATPases as a therapeutic target in a variety of human diseases.

Validation of the Target Protein of Insecticidal Dihydroagarofuran Sesquiterpene Polyesters

A series of insecticidal dihydroagarofuran sesquiterpene polyesters were isolated from the root bark of Chinese bittersweet (Celastrus angulatus Max). A previous study indicated that these compounds affect the digestive system of insects, and aminopeptidase N3 and V-ATPase have been identified as the most putative target proteins by affinity chromatography. In this study, the correlation between the affinity of the compounds to subunit H and the insecticidal activity or inhibitory effect on the activity of V-ATPase was analyzed to validate the target protein. Results indicated that the subunit H of V-ATPase was the target protein of the insecticidal compounds. In addition, the possible mechanism of action of the compounds was discussed. The results provide new ideas for developing pesticides acting on V-ATPase of insects.

Diet-Delivered dsRNAs for Juvenile Hormone-Binding Protein and Vacuolar ATPase-H Implied Their Potential in the Management of the Melon Aphid (Hemiptera: Aphididae)

RNA interference is a sequence-specific gene silencing mechanism mediated by double-stranded RNA (dsRNA), which has been harnessed as a useful tool in devising novel insect pest management strategies for various pests such as melon aphid, Aphis gossypii (Glover). In the current study, we cloned and sequenced juvenile hormone-binding protein (JHBP) and vacuolar ATPase subunit H (V-ATPase-H) from A. gossypii. We also showed the effectiveness of diet-mediated delivery of dsRNA for JHBP and V-ATPase-H, which silenced the above genes and resulted in mortality. The extent of silencing and mortality were similar for both genes up until 96 h. Bioassay results revealed that the target genes were silenced variably, 1.0 µg/µl concentration having a more profound effect than 0.5 and 0.25 µg/µl concentration in reducing the cognate mRNA transcript level. Results indicated a 9.56–73.21% down regulation (across time and concentrations for both the genes) that resulted in the mortality of A. gossypii. Mortality was in the range of 10–63% for both these genes. Thus, the current study demonstrated the potentiality of both JHBP and V-ATPase-H as excellent targets for the management of A. gossypii.

Expression and activity of V-H+ -ATPase in gill and kidney of marbled eel Anguilla marmorata in response to salinity challenge

The full-length complementary (c)DNA of vacuolar-type-H(+) -ATPase B1 gene (vhab1) in marbled eel Anguilla marmorata with 1741 base pairs (bp) was identified. It contained a 1512 bp open reading frame encoding a polypeptide with 503 amino acids (55·9 kDa), an 83 bp 5'-untranslated region (UTR) and a 146 bp 3'-UTR. The expression levels of A. marmorata vhab1 in gill and kidney of A. marmorata were evaluated at different intervals during the exposure to various salinities (0, 10 and 25). The results indicated that the expression levels of A. marmorata vhab1 messenger (m)RNA in gill and kidney had a significant increase and reached the highest level at 1 h in brackish water (BW, salinity 10) group and 6 h in seawater (SW, salinity 25) group. Therefore, salinity did affect the relative expression level of A. marmorata vhab1 mRNA in gills, which exhibited the enhancement by c. 44 times in SW group when compared with that in fresh water. No remarkable difference in the expression of A. marmorata vhab1 mRNA was observed after 15 days of SW exposure (P > 0·05). V-H(+) -ATPase activity exhibited an increase by two- to three-fold when compared with that in gill and kidney from the control group. The consequence primarily suggested that A. marmorata vhab1 gene product in elvers from A. marmorata plays an important role in adaptation response to SW.

Protein-protein interactions within the ensemble, eukaryotic V-ATPase, and its concerted interactions with cellular machineries

The V1VO-ATPase (V-ATPase) is the important proton-pump in eukaryotic cells, responsible for pH-homeostasis, pH-sensing and amino acid sensing, and therefore essential for cell growths and metabolism. ATP-cleavage in the catalytic A3B3-hexamer of V1 has to be communicated via several so-called central and peripheral stalk units to the proton-pumping VO-part, which is membrane-embedded. A unique feature of V1VO-ATPase regulation is its reversible disassembly of the V1 and VO domain. Actin provides a network to hold the V1 in proximity to the VO, enabling effective V1VO-assembly to occur. Besides binding to actin, the 14-subunit V-ATPase interacts with multi-subunit machineries to form cellular sensors, which regulate the pH in cellular compartments or amino acid signaling in lysosomes. Here we describe a variety of subunit-subunit interactions within the V-ATPase enzyme during catalysis and its protein-protein assembling with key cellular machineries, essential for cellular function.

Crystal structure of subunits D and F in complex gives insight into energy transmission of the eukaryotic V-ATPase from Saccharomyces cerevisiae

Eukaryotic V1VO-ATPases hydrolyze ATP in the V1 domain coupled to ion pumping in VO. A unique mode of regulation of V-ATPases is the reversible disassembly of V1 and VO, which reduces ATPase activity and causes silencing of ion conduction. The subunits D and F are proposed to be key in these enzymatic processes. Here, we describe the structures of two conformations of the subunit DF assembly of Saccharomyces cerevisiae (ScDF) V-ATPase at 3.1 Å resolution. Subunit D (ScD) consists of a long pair of α-helices connected by a short helix ((79)IGYQVQE(85)) as well as a β-hairpin region, which is flanked by two flexible loops. The long pair of helices is composed of the N-terminal α-helix and the C-terminal helix, showing structural alterations in the two ScDF structures. The entire subunit F (ScF) consists of an N-terminal domain of four β-strands (β1-β4) connected by four α-helices (α1-α4). α1 and β2 are connected via the loop (26)GQITPETQEK(35), which is unique in eukaryotic V-ATPases. Adjacent to the N-terminal domain is a flexible loop, followed by a C-terminal α-helix (α5). A perpendicular and extended conformation of helix α5 was observed in the two crystal structures and in solution x-ray scattering experiments, respectively. Fitted into the nucleotide-bound A3B3 structure of the related A-ATP synthase from Enterococcus hirae, the arrangements of the ScDF molecules reflect their central function in ATPase-coupled ion conduction. Furthermore, the flexibility of the terminal helices of both subunits as well as the loop (26)GQITPETQEK(35) provides information about the regulatory step of reversible V1VO disassembly.

The study of vacuolar-type ATPases by single particle electron microscopy

Nature’s molecular machines often work through the concerted action of many different protein subunits, which can give rise to large structures with complex activities. Vacuolar-type ATPases (V-ATPases) are membrane-embedded protein assemblies with a unique rotary catalytic mechanism. The dynamic nature and instability of V-ATPases make structural and functional studies of these enzymes challenging. Electron microscopy (EM) techniques, especially single particle electron cryomicroscopy (cryo-EM) and negative-stain EM, have provided extensive insight into the structure and function of these protein complexes. This minireview outlines what has been learned about V-ATPases using electron microscopy, highlights current challenges for their structural study, and discusses what cryo-EM will allow us to learn about these fascinating enzymes in the future.

Cloning, expression and purification of subunit H of vacuolar H⁺-ATPase from Mythimna separata Walker (Lepidoptera: Noctuidae)

The vacuolar (H+)-ATPase (V-ATPase) of insect, which is composed of membrane-bound V0 complex and peripheral V1 complex, participates in lots of important physiological process. Subunit H, as a subunit of V1 complex, plays a vital role in bridging the communication between V1 and V0 complexes and interaction with other proteins. Yeast subunit H has been successfully crystallized through expression in E. coli, but little is known about the structure of insect subunit H. In this study, we cloned, expressed and purified the subunit H from midgut of Mythimna separata Walker. Through RACE (rapidly amplification of cDNA ends) technique, we got 1807 bp full length of subunit H, and to keep the nature structure of subunit H, we constructed Baculovirus expression vector with His-tag in the C-terminal and expressed the recombinant protein in insect sf9 cells, thereafter, purified the recombinant protein by Ni-NTA columns. Results of SDS-PAGE, western blotting and mass spectrometry showed that the recombinant protein was successfully expressed. The method of expressing and purifying M. separata subunit H will provide a foundation for obtaining the crystal of subunit H and further study of the design of novel insecticides based on its structure and function.

A conserved cysteine residue of Bacillus subtilis SpoIIIJ is important for endospore development

During sporulation in Bacillus subtilis, the onset of activity of the late forespore-specific sigma factor σ^G coincides with completion of forespore engulfment by the mother cell. At this stage, the forespore becomes a free protoplast, surrounded by the mother cell cytoplasm and separated from it by two membranes that derive from the asymmetric division septum. Continued gene expression in the forespore, isolated from the surrounding medium, relies on the SpoIIIA-SpoIIQ secretion system assembled from proteins synthesised both in the mother cell and in the forespore. The membrane protein insertase SpoIIIJ, of the YidC/Oxa1/Alb3 family, is involved in the assembly of the SpoIIIA-SpoIIQ complex. Here we show that SpoIIIJ exists as a mixture of monomers and dimers stabilised by a disulphide bond. We show that residue Cys134 within transmembrane segment 2 (TM2) of SpoIIIJ is important to stabilise the protein in the dimeric form. Labelling of Cys134 with a Cys-reactive reagent could only be achieved under stringent conditions, suggesting a tight association at least in part through TM2, between monomers in the membrane. Substitution of Cys134 by an Ala results in accumulation of the monomer, and reduces SpoIIIJ function in vivo. Therefore, SpoIIIJ activity in vivo appears to require dimer formation.

Saccharomyces cerevisiae vacuolar H+-ATPase regulation by disassembly and reassembly: one structure and multiple signals

Vacuolar H(+)-ATPases (V-ATPases) are highly conserved ATP-driven proton pumps responsible for acidification of intracellular compartments. V-ATPase proton transport energizes secondary transport systems and is essential for lysosomal/vacuolar and endosomal functions. These dynamic molecular motors are composed of multiple subunits regulated in part by reversible disassembly, which reversibly inactivates them. Reversible disassembly is intertwined with glycolysis, the RAS/cyclic AMP (cAMP)/protein kinase A (PKA) pathway, and phosphoinositides, but the mechanisms involved are elusive. The atomic- and pseudo-atomic-resolution structures of the V-ATPases are shedding light on the molecular dynamics that regulate V-ATPase assembly. Although all eukaryotic V-ATPases may be built with an inherent capacity to reversibly disassemble, not all do so. V-ATPase subunit isoforms and their interactions with membrane lipids and a V-ATPase-exclusive chaperone influence V-ATPase assembly. This minireview reports on the mechanisms governing reversible disassembly in the yeast Saccharomyces cerevisiae, keeping in perspective our present understanding of the V-ATPase architecture and its alignment with the cellular processes and signals involved.

Eukaryotic V-ATPase: novel structural findings and functional insights

The eukaryotic V-type adenosine triphosphatase (V-ATPase) is a multi-subunit membrane protein complex that is evolutionarily related to F-type adenosine triphosphate (ATP) synthases and A-ATP synthases. These ATPases/ATP synthases are functionally conserved and operate as rotary proton-pumping nano-motors, invented by Nature billions of years ago. In the first part of this review we will focus on recent structural findings of eukaryotic V-ATPases and discuss the role of different subunits in the function of the V-ATPase holocomplex. Despite structural and functional similarities between rotary ATPases, the eukaryotic V-ATPases are the most complex enzymes that have acquired some unconventional cellular functions during evolution. In particular, the novel roles of V-ATPases in the regulation of cellular receptors and their trafficking via endocytotic and exocytotic pathways were recently uncovered. In the second part of this review we will discuss these unique roles of V-ATPases in modulation of function of cellular receptors, involved in the development and progression of diseases such as cancer and diabetes as well as neurodegenerative and kidney disorders. Moreover, it was recently revealed that the V-ATPase itself functions as an evolutionarily conserved pH sensor and receptor for cytohesin-2/Arf-family GTP-binding proteins. Thus, in the third part of the review we will evaluate the structural basis for and functional insights into this novel concept, followed by the analysis of the potentially essential role of V-ATPase in the regulation of this signaling pathway in health and disease. Finally, future prospects for structural and functional studies of the eukaryotic V-ATPase will be discussed.

Subunit positioning and stator filament stiffness in regulation and power transmission in the V1 motor of the Manduca sexta V-ATPase

The vacuolar H⁺-ATPase (V-ATPase) is an ATP-driven proton pump essential to the function of eukaryotic cells. Its cytoplasmic V₁ domain is an ATPase, normally coupled to membrane-bound proton pump V_o via a rotary mechanism. How these asymmetric motors are coupled remains poorly understood. Low energy status can trigger release of V₁ from the membrane and curtail ATP hydrolysis. To investigate the molecular basis for these processes, we have carried out cryo-electron microscopy three-dimensional reconstruction of deactivated V₁ from Manduca sexta. In the resulting model, three peripheral stalks that are parts of the mechanical stator of the V-ATPase are clearly resolved as unsupported filaments in the same conformations as in the holoenzyme. They are likely therefore to have inherent stiffness consistent with a role as flexible rods in buffering elastic power transmission between the domains of the V-ATPase. Inactivated V₁ adopted a homogeneous resting state with one open active site adjacent to the stator filament normally linked to the H subunit. Although present at 1:1 stoichiometry with V₁, both recombinant subunit C reconstituted with V₁ and its endogenous subunit H were poorly resolved in three-dimensional reconstructions, suggesting structural heterogeneity in the region at the base of V₁ that could indicate positional variability. If the position of H can vary, existing mechanistic models of deactivation in which it binds to and locks the axle of the V-ATPase rotary motor would need to be re-evaluated.

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Dissociation of vacuolar H⁺-ATPase domains deactivates its V₁ motor.

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V₁ has one “open” catalytic site linked to the stator filament bound by subunit H.

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Movement of subunit H to prevent rotary catalysis is possible.

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Three stator filaments project from deactivated V₁, indicating inherent stiffness.

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This work gives new insight into energetic coupling and control in V-ATPases.

Control of cell growth: Rag GTPases in activation of TORC1

The target of rapamycin (TOR) is a central regulator controlling cell growth. TOR is highly conserved from yeast to mammals, and is deregulated in human cancers and diabetes. TOR complex 1 (TORC1) integrates signals from growth factors, cellular energy status, stress, and amino acids to control cell growth, mitochondrial metabolism, and lipid biosynthesis. The mechanisms of growth factors and cellular energy status in regulating TORC1 have been well established, whereas the mechanism by which amino acid induces TORC1 remains largely unknown. Recent studies revealed that Rag GTPases play a central role in the regulation of TORC1 activation in response to amino acids. In this review, we will discuss the recent progress in our understanding of Rag GTPase-regulated TORC1 activation in response to amino acids. Particular focus will be given to the function of Rag GTPases in TORC1 activation and how Rag GTPases are regulated by amino acids.

Structural analysis of the N-terminal domain of subunit a of the yeast vacuolar ATPase (V-ATPase) using accessibility of single cysteine substitutions to chemical modification

The vacuolar ATPase (V-ATPase) is a multisubunit complex that carries out ATP-driven proton transport. It is composed of a peripheral V1 domain that hydrolyzes ATP and an integral V0 domain that translocates protons. Subunit a is a 100-kDa integral membrane protein (part of V0) that possesses an N-terminal cytoplasmic domain and a C-terminal hydrophobic domain. Although the C-terminal domain functions in proton transport, the N-terminal domain is critical for intracellular targeting and regulation of V-ATPase assembly. Despite its importance, there is currently no high resolution structure for subunit a of the V-ATPase. Recently, the crystal structure of the N-terminal domain of the related subunit I from the archaebacterium Meiothermus ruber was reported. We have used homology modeling to construct a model of the N-terminal domain of Vph1p, one of two isoforms of subunit a expressed in yeast. To test this model, unique cysteine residues were introduced into a Cys-less form of Vph1p and their accessibility to modification by the sulfhydryl reagent 3-(N-maleimido-propionyl) biocytin (MPB) was determined. In addition, accessibility of introduced cysteine residues to MPB modification was compared in the V1V0 complex and the free V0 domain to identify residues protected from modification by the presence of V1. The results provide an experimental test of the proposed model and have identified regions of the N-terminal domain of subunit a that likely serve as interfacial contact sites with the peripheral V1 domain. The possible significance of these results for in vivo regulation of V-ATPase assembly is discussed.

Crystal and NMR structures give insights into the role and dynamics of subunit F of the eukaryotic V-ATPase from Saccharomyces cerevisiae

Subunit F of V-ATPases is proposed to undergo structural alterations during catalysis and reversible dissociation from the V1VO complex. Recently, we determined the low resolution structure of F from Saccharomyces cerevisiae V-ATPase, showing an N-terminal egg shape, connected to a C-terminal hook-like segment via a linker region. To understand the mechanistic role of subunit F of S. cerevisiae V-ATPase, composed of 118 amino acids, the crystal structure of the major part of F, F(1-94), was solved at 2.3 Å resolution. The structural features were confirmed by solution NMR spectroscopy using the entire F subunit. The eukaryotic F subunit consists of the N-terminal F(1-94) domain with four-parallel β-strands, which are intermittently surrounded by four α-helices, and the C terminus, including the α5-helix encompassing residues 103 to 113. Two loops (26)GQITPETQEK(35) and (60)ERDDI(64) are described to be essential in mechanistic processes of the V-ATPase enzyme. The (26)GQITPETQEK(35) loop becomes exposed when fitted into the recently determined EM structure of the yeast V1VO-ATPase. A mechanism is proposed in which the (26)GQITPETQEK(35) loop of subunit F and the flexible C-terminal domain of subunit H move in proximity, leading to an inhibitory effect of ATPase activity in V1. Subunits D and F are demonstrated to interact with subunit d. Together with NMR dynamics, the role of subunit F has been discussed in the light of its interactions in the processes of reversible disassembly and ATP hydrolysis of V-ATPases by transmitting movements of subunit d and H of the VO and V1 sector, respectively.

Elevated expression of the V-ATPase C subunit triggers JNK-dependent cell invasion and overgrowth in a Drosophila epithelium

The C subunit of the vacuolar H⁺-ATPase or V-ATPase regulates the activity and assembly of the proton pump at cellular membranes. It has been shown to be strongly upregulated in oral squamous cell carcinoma, a highly metastatic epithelial cancer. In addition, increased V-ATPase activity appears to correlate with invasiveness of cancer cells, but the underlying mechanism is largely unknown. Using the Drosophila wing imaginal epithelium as an in vivo model system, we demonstrate that overexpression of Vha44, the Drosophila orthologue of the C subunit, causes a tumor-like tissue transformation in cells of the wing epithelium. Overexpressing cells are excluded from the epithelium and acquire invasive properties while displaying high apoptotic rates. Blocking apoptosis in these cells unmasks a strong proliferation stimulus, leading to overgrowth. Furthermore, we show that excess Vha44 greatly increases acidification of endocytic compartments and interferes with endosomal trafficking. As a result, cargoes such as GFP-Lamp1 and Notch accumulate in highly acidified enlarged endolysosomal compartments. Consistent with previous reports on the endocytic activation of Eiger/JNK signaling, we find that V-ATPase stimulation by Vha44 causes JNK signaling activation whereas downmodulation of JNK signaling rescues the invasive phenotypes. In summary, our in vivo-findings demonstrate that increased levels of V-ATPase C subunit induce a Eiger/JNK-dependent cell transformation within an epithelial organ that recapitulates early carcinoma stages.

Extracellular glucose increases the coupling capacity of the yeast V H+-ATPase and the resistance of its H+ transport activity to nitrate inhibition

V H(+)-ATPase has an important role in a variety of key physiological processes. This enzyme is reversibly activated/partly inactivated by the addition/exhaustion of extracellular glucose. The current model of its regulation assumes the reversible disassembly/reassembly of ∼60-70% of the V1 and V0 membrane complexes, which are responsible for ATP hydrolysis and H(+) conductance, respectively. The number of assembled complexes determines the pump activity because disassembled complexes are inactive. The model predicts the identical catalytic properties for the activated and semi-active enzymes molecules. To verify the model predictions we have isolated total membranes from yeast spheroplasts that were pre-incubated either with or without glucose. Nitrate treatment of membranes revealed the similar ATPase inhibition for two enzyme states, suggesting that they have identical structures that are essential for ATP hydrolysis. However, H(+) transport was inhibited more than the ATPase activities, indicating a nitrate uncoupling action, which was significantly higher for the nonactivated enzyme. This finding suggests that the structure of the non-activated enzyme, which is essential for H(+) transport, is less stable than that of the activated enzyme. Moreover, the glucose activation of the pump increases i) its coupling capacity; ii) its K(M) for ATP hydrolysis and ATP affinity for H(+) transport; iii) the Vmax for H(+) transport in comparison with the Vmax for ATP hydrolysis and iv) the immune reactivity of catalytic subunit A and regulatory subunit B by 9.3 and 2.4 times, respectively. The protein content of subunits A and B was not changed by extracellular glucose. We propose that instead of the dissociation/reassociation of complexes V1 and V0, changes in the extracellular glucose concentration cause reversible and asymmetrical modulations in the immune reactivity of subunits A and B by their putative biochemical modifications. This response asymmetrically modulates H(+)-transport and ATP hydrolysis, exhibiting distinct properties for the activated versus non-activated enzymes.

An update in the structure, function, and regulation of V-ATPases: the role of the C subunit

Vacuolar ATPases (V-ATPases) are present in specialized proton secretory cells in which they pump protons across the membranes of various intracellular organelles and across the plasma membrane. The proton transport mechanism is electrogenic and establishes an acidic pH and a positive transmembrane potential in these intracellular and extracellular compartments. V-ATPases have been found to be practically identical in terms of the composition of their subunits in all eukaryotic cells. They have two distinct structures: a peripheral catalytic sector (V1) and a hydrophobic membrane sector (V0) responsible for driving protons. V-ATPase activity is regulated by three different mechanisms, which control pump density, association/dissociation of the V1 and V0 domains, and secretory activity. The C subunit is a 40-kDa protein located in the V1 domain of V-ATPase. The protein is encoded by the ATP6V1C gene and is located at position 22 of the long arm of chromosome 8 (8q22.3). The C subunit has very important functions in terms of controlling the regulation of the reversible dissociation of V-ATPases.

Structure of the vacuolar-type ATPase from Saccharomyces cerevisiae at 11-Å resolution

Vacuolar-type ATPases (V-type ATPases) in eukaryotic cells are large membrane protein complexes that acidify various intracellular compartments. The enzymes are regulated by dissociation of the V(1) and V(O) regions of the complex. Here we present the structure of the Saccharomyces cerevisiae V-type ATPase at 11-Å resolution by cryo-EM of protein particles in ice. The structure explains many cross-linking and protein interaction studies. Docking of crystal structures suggests that inhibition of ATPase activity by the dissociated V(1) region involves rearrangement of the N- and C-terminal domains of subunit H and also suggests how this inhibition is triggered upon dissociation. We provide support for this model by demonstrating that mutation of subunit H to increase the rigidity of the linker between its two domains decreases its ability to inhibit ATPase activity.

Probing subunit-subunit interactions in the yeast vacuolar ATPase by peptide arrays

BACKGROUND

Vacuolar (H(+))-ATPase (V-ATPase; V(1)V(o)-ATPase) is a large multisubunit enzyme complex found in the endomembrane system of all eukaryotic cells where its proton pumping action serves to acidify subcellular organelles. In the plasma membrane of certain specialized tissues, V-ATPase functions to pump protons from the cytoplasm into the extracellular space. The activity of the V-ATPase is regulated by a reversible dissociation mechanism that involves breaking and re-forming of protein-protein interactions in the V(1)-ATPase - V(o)-proton channel interface. The mechanism responsible for regulated V-ATPase dissociation is poorly understood, largely due to a lack of detailed knowledge of the molecular interactions that are responsible for the structural and functional link between the soluble ATPase and membrane bound proton channel domains.

METHODOLOGY/PRINCIPAL FINDINGS

To gain insight into where some of the stator subunits of the V-ATPase associate with each other, we have developed peptide arrays from the primary sequences of V-ATPase subunits. By probing the peptide arrays with individually expressed V-ATPase subunits, we have identified several key interactions involving stator subunits E, G, C, H and the N-terminal domain of the membrane bound a subunit.

CONCLUSIONS

The subunit-peptide interactions identified from the peptide arrays complement low resolution structural models of the eukaryotic vacuolar ATPase obtained from transmission electron microscopy. The subunit-subunit interaction data are discussed in context of our current model of reversible enzyme dissociation.

Crystallization and preliminary X-ray crystallographic analysis of subunit F (F(1-94)), an essential coupling subunit of the eukaryotic V(1)V(O)-ATPase from Saccharomyces cerevisiae

V-ATPases are very complex multi-subunit enzymes which function as proton-pumping rotary nanomotors. The rotary and coupling subunit F (F(1-94)) was crystallized by the hanging-drop vapour-diffusion method. The native crystals diffracted to a resolution of 2.64 Å and belonged to space group C222(1), with unit-cell parameters a = 47.21, b = 160.26, c = 102.49 Å. The selenomethionyl form of the F(1-94) I69M mutant diffracted to a resolution of 2.3 Å and belonged to space group C222(1), with unit-cell parameters a = 47.22, b = 160.83, c = 102.74 Å. Initial phasing and model building suggested the presence of four molecules in the asymmetric unit.

Vacuolar ATPase subunit H is essential for the survival and moulting of Locusta migratoria manilensis

Vacuolar (H(+) )-ATPase (V-ATPase) functions as an electrogenic pump, transporting protons from the cytoplasm to the extracellular fluid to generate cell-negative membrane voltage. The V-ATPase subunit H, encoded by Vhasfd, is required for V-ATPase activity. In this study, the gene encoding V-ATPase subunit H from Locusta migratoria manilensis was cloned, and designated as Lm-Vhasfd. The complete cDNA sequence is 2018 bp, with an open reading frame encoding 515 amino acid residues. Semi-quantitative reverse transcription PCR (RT-PCR) showed that Lm-Vhasfd transcription is high in the haemolymph, midgut, trunk and leg, but relatively low in the fat body and head tissues. Injection with a specific double-strand RNA (dsRNA) led to a significant decrease in Lm-Vhasfd mRNA, V-ATPase enzyme activity and ATP concentration. Bioassays showed that silencing Lm-Vhasfd led to the death of individuals and various moulting defects. The accumulative mortality of the RNA interference (RNAi) mutant 11 days post-injection was 96.7%, which was conspicuously higher than that seen in wild type locusts. These RNAi phenotypes demonstrate that Lm-Vhasfd is essential for the growth and moulting of L. migratoria manilensis.

V-ATPases in osteoclasts: structure, function and potential inhibitors of bone resorption

The vacuolar-type H(+)-ATPase (V-ATPase) proton pump is a macromolecular complex composed of at least 14 subunits organized into two functional domains, V(1) and V(0). The complex is located on the ruffled border plasma membrane of bone-resorbing osteoclasts, mediating extracellular acidification for bone demineralization during bone resorption. Genetic studies from mice to man implicate a critical role for V-ATPase subunits in osteoclast-related diseases including osteopetrosis and osteoporosis. Thus, the V-ATPase complex is a potential molecular target for the development of novel anti-resorptive agents useful for the treatment of osteolytic diseases. Here, we review the current structure and function of V-ATPase subunits, emphasizing their exquisite roles in osteoclastic function. In addition, we compare several distinct classes of V-ATPase inhibitors with specific inhibitory effects on osteoclasts. Understanding the structure-function relationship of the osteoclast V-ATPase may lead to the development of osteoclast-specific V-ATPase inhibitors that may serve as alternative therapies for the treatment of osteolytic diseases.

The blowfly salivary gland - a model system for analyzing the regulation of plasma membrane V-ATPase

Vacuolar H(+)-ATPases (V-ATPases) are heteromultimeric proteins that use the energy of ATP hydrolysis for the electrogenic transport of protons across membranes. They are common to all eukaryotic cells and are located in the plasma membrane or in membranes of acid organelles. In many insect epithelia, V-ATPase molecules reside in large numbers in the apical plasma membrane and create an electrochemical proton gradient that is used for the acidification or alkalinization of the extracellular space, the secretion or reabsorption of ions and fluids, the import of nutrients, and diverse other cellular activities. Here, we summarize our results on the functions and regulation of V-ATPase in the tubular salivary gland of the blowfly Calliphora vicina. In this gland, V-ATPase activity energizes the secretion of a KCl-rich saliva in response to the neurohormone serotonin (5-HT). Because of particular morphological and physiological features, the blowfly salivary glands are a superior and exemplary system for the analysis of the intracellular signaling pathways and mechanisms that modulate V-ATPase activity and solute transport in an insect epithelium.

Targeting reversible disassembly as a mechanism of controlling V-ATPase activity

Vacuolar proton-translocating ATPases (V-ATPases) are highly conserved proton pumps consisting of a peripheral membrane subcomplex called V1, which contains the sites of ATP hydrolysis, attached to an integral membrane subcomplex called Vo, which encompasses the proton pore. V-ATPase regulation by reversible dissociation, characterized by release of assembled V1 sectors into the cytosol and inhibition of both ATPase and proton transport activities, was first identified in tobacco hornworm and yeast. It has since become clear that modulation of V-ATPase assembly level is also a regulatory mechanism in mammalian cells. In this review, the implications of reversible disassembly for V-ATPase structure are discussed, along with insights into underlying subunit-subunit interactions provided by recent structural work. Although initial experiments focused on glucose deprivation as a trigger for disassembly, it is now clear that V-ATPase assembly can be regulated by other extracellular conditions. Consistent with a complex, integrated response to extracellular signals, a number of different regulatory proteins, including RAVE/rabconnectin, aldolase and other glycolytic enzymes, and protein kinase A have been suggested to control V-ATPase assembly and disassembly. It is likely that multiple signaling pathways dictate the ultimate level of assembly and activity. Tissue-specific V-ATPase inhibition is a potential therapy for osteoporosis and cancer; the possibility of exploiting reversible disassembly in design of novel V-ATPase inhibitors is discussed.

Decreased expression of ATP6V1H in type 2 diabetes: a pilot report on the diabetes risk study in Mexican Americans

OBJECTIVE

Previous studies in mice and humans observed down-regulation of the gene expression of ATP6V1H associated with type 2 diabetes. This study identified prospectively changes in ATP6V1H expression before and after overt diabetes.

METHODS

Expression of ATP6V1H in peripheral blood was compared pre and post development of diabetes in nine individuals.

RESULTS

Considerable variation of ATP6V1H mRNA levels was observed between different individuals. However, within each individual the decrease in expression of ATP6V1H with the development of diabetes was highly statistically significant.

CONCLUSIONS

ATP6V1H may represent a critical molecular mechanism involved in the development of type 2 diabetes and its compilations through its important regulatory effect on vacuolar-ATPase activity.

Structural divergence of the rotary ATPases

The rotary ATPase family of membrane protein complexes may have only three members, but each one plays a fundamental role in biological energy conversion. The F1Fo-ATPase (F-ATPase) couples ATP synthesis to the electrochemical membrane potential in bacteria, mitochondria and chloroplasts, while the vacuolar H+-ATPase (V-ATPase) operates as an ATP-driven proton pump in eukaryotic membranes. In different species of archaea and bacteria, the A1Ao-ATPase (A-ATPase) can function as either an ATP synthase or an ion pump. All three of these multi-subunit complexes are rotary molecular motors, sharing a fundamentally similar mechanism in which rotational movement drives the energy conversion process. By analogy to macroscopic systems, individual subunits can be assigned to rotor, axle or stator functions. Recently, three-dimensional reconstructions from electron microscopy and single particle image processing have led to a significant step forward in understanding of the overall architecture of all three forms of these complexes and have allowed the organisation of subunits within the rotor and stator parts of the motors to be more clearly mapped out. This review describes the emerging consensus regarding the organisation of the rotor and stator components of V-, A- and F-ATPases, examining core similarities that point to a common evolutionary origin, and highlighting key differences. In particular, it discusses how newly revealed variation in the complexity of the inter-domain connections may impact on the mechanics and regulation of these molecular machines.

Structural bases of physiological functions and roles of the vacuolar H(+)-ATPase

Vacuolar-type H(+)-ATPases (V-ATPases) is a large multi-protein complex containing at least 14 different subunits, in which subunits A, B, C, D, E, F, G, and H compose the peripheral 500-kDa V(1) responsible for ATP hydrolysis, and subunits a, c, c', c″, and d assembly the 250-kDa membrane-integral V(0) harboring the rotary mechanism to transport protons across the membrane. The assembly of V-ATPases requires the presence of all V(1) and V(0) subunits, in which the V(1) must be completely assembled prior to association with the V(0), accordingly the V(0) failing to assemble cannot provide a membrane anchor for the V(1), thereby prohibiting membrane association of the V-ATPase subunits. The V-ATPase mediates acidification of intracellular compartments and regulates diverse critical physiological processes of cell for functions of its numerous functional subunits. The core catalytic mechanism of the V-ATPase is a rotational catalytic mechanism. The V-ATPase holoenzyme activity is regulated by the reversible assembly/disassembly of the V(1) and V(0), the targeting and recycling of V-ATPase-containing vesicles to and from the plasma membrane, the coupling ratio between ATP hydrolysis and proton pumping, ATP, Ca(2+), and its inhibitors and activators.

Solution structure of subunit F (Vma7p) of the eukaryotic V(1)V(O) ATPase from Saccharomyces cerevisiae derived from SAXS and NMR spectroscopy

Vacuolar ATPases use the energy derived from ATP hydrolysis, catalyzed in the A(3)B(3) sector of the V(1) ATPase to pump protons via the membrane-embedded V(O) sector. The energy coupling between the two sectors occurs via the so-called central stalk, to which subunit F does belong. Here we present the first low resolution structure of recombinant subunit F (Vma7p) of a eukaryotic V-ATPase from Saccharomyces cerevisiae, analyzed by small angle X-ray scattering (SAXS). The protein is divided into a 5.5nm long egg-like shaped region, connected via a 1.5nm linker to a hook-like segment at one end. Circular dichroism spectroscopy revealed that subunit F comprises of 43% α-helix, 32% β-sheet and a 25% random coil arrangement. To determine the localization of the N- and C-termini in the protein, the C-terminal truncated form of F, F(1-94) was produced and analyzed by SAXS. Comparison of the F(1-94) shape with the one of subunit F showed the missing hook-like region in F(1-94), supported by the decreased D(max) value of F(1-94) (7.0nm), and indicating that the hook-like region consists of the C-terminal residues. The NMR solution structure of the C-terminal peptide, F(90-116), was solved, displaying an α-helical region between residues 103 and 113. The F(90-116) solution structure fitted well in the hook-like region of subunit F. Finally, the arrangement of subunit F within the V(1) ATPase is discussed.

Regulation and isoform function of the V-ATPases

The vacuolar (H(+))-ATPases are ATP-dependent proton pumps that acidify intracellular compartments and, in some cases, transport protons across the plasma membrane of eukaryotic cells. Intracellular V-ATPases play an important role in normal physiological processes such as receptor-mediated endocytosis, intracellular membrane trafficking, pro-hormone processing, protein degradation, and the coupled uptake of small molecules, such as neurotransmitters. They also function in the entry of various pathogenic agents, including many envelope viruses, like influenza virus, and toxins, like anthrax toxin. Plasma membrane V-ATPases function in renal pH homeostasis, bone resorption and sperm maturation, and various disease processes, including renal tubular acidosis, osteopetrosis, and tumor metastasis. V-ATPases are composed of a peripheral V(1) domain containing eight different subunits that is responsible for ATP hydrolysis and an integral V(0) domain containing six different subunits that translocates protons. In mammalian cells, most of the V-ATPase subunits exist in multiple isoforms which are often expressed in a tissue specific manner. Isoforms of one of the V(0) subunits (subunit a) have been shown to possess information that targets the V-ATPase to distinct cellular destinations. Mutations in isoforms of subunit a lead to the human diseases osteopetrosis and renal tubular acidosis. A number of mechanisms are employed to regulate V-ATPase activity in vivo, including reversible dissociation of the V(1) and V(0) domains, control of the tightness of coupling of proton transport and ATP hydrolysis, and selective targeting of V-ATPases to distinct cellular membranes. Isoforms of subunit a are involved in regulation both via the control of coupling and via selective targeting. This review will begin with a brief introduction to the function, structure, and mechanism of the V-ATPases followed by a discussion of the role of V-ATPase subunit isoforms and the mechanisms involved in regulation of V-ATPase activity.

The NMR solution structure of subunit G (G(61)(-)(101)) of the eukaryotic V1VO ATPase from Saccharomyces cerevisiae

Subunit G is an essential stalk subunit of the eukaryotic proton pump V(1)V(O) ATPase. Previously the structure of the N-terminal region, G(1)(-)(59), of the 13kDa subunit G was solved at higher resolution. Here solution NMR was performed to determine the structure of the recombinant C-terminal region (G(61)(-)(101)) of subunit G of the Saccharomyces cerevisiae V(1)V(O) ATPase. The protein forms an extended alpha-helix between residues 64 and 100, whereby the first five- and the last residues of G(61)(-)(101) are flexible. The surface charge distribution of G(61)(-)(101) reveals an amphiphilic character at the C-terminus due to positive and negative charge distribution at one side and a hydrophobic surface on the opposite side of the structure. The hydrophobic surface pattern is mainly formed by alanine residues. The alanine residues 72, 74 and 81 were exchanged by a single cysteine in the entire subunit G. Cysteines at positions 72 and 81 showed disulfide formation. In contrast, no crosslink could be formed for the mutant Ala74Cys. Together with the recently determined NMR solution structure of G(1)(-)(59), the presented solution structure of G(61)(-)(101) enabled us to present a first structural model of the entire subunit G of the S. cerevisiae V(1)V(O) ATPase.

Cellular physiology of the renal H+ATPase

PURPOSE OF REVIEW

Vacuolar-type H+ATPases are multisubunit macromolecules that play an essential role in renal acid-base homeostasis. Other cellular processes also rely on the proton pumping ability of H+ATPases to acidify organellar or lumenal spaces. Several diseases, including distal renal tubular acidosis, osteoporosis and wrinkly skin syndrome, are due to mutations in genes encoding alternate subunits that make up the H+ATPase. This review highlights recent key articles in this research area.

RECENT FINDINGS

Further insights into the structure, expression and regulation of H+ATPases have been elucidated, within the kidney and elsewhere. This knowledge may enhance the potential for future drug targeting.

SUMMARY

Novel findings concerning tissue-specific subunits of the H+ATPase that are important in the kidney and more general lessons of H+ATPase function and regulation are slowly emerging, though the paucity of cellular tools available has to date limited progress.