V-ATPase Disassembly at the Yeast Lysosome-Like Vacuole Is a Phenotypic Driver of Lysosome Dysfunction in Replicative Aging.

Declines in lysosomal acidification and function with aging are observed in organisms ranging from yeast to humans. V-ATPases play a central role in organelle acidification and V-ATPase activity is regulated by reversible disassembly in many different settings. Using the yeast Saccharomyces cerevisiae as a replicative aging model, we demonstrate that V-ATPases disassemble into their V 1 and V 0 subcomplexes in aging cells, with release of V 1 subunit C (Vma5) from the lysosome-like vacuole into the cytosol. Disassembly is observed after > 5 cell divisions and results in overall vacuole alkalinization. Caloric restriction, an established mechanism for reversing many age-related outcomes, prevents V-ATPase disassembly in older cells and preserves vacuolar pH homeostasis. Reversible disassembly is controlled in part by the activity of two opposing and conserved factors, the RAVE complex and Oxr1. The RAVE complex promotes V-ATPase assembly and a rav1 Δ mutant shortens replicative lifespan; Oxr1 promotes disassembly and an oxr1 Δ mutation extends lifespan. Importantly, the level of Rav2, a key subunit of the RAVE complex, declines in aged cells. These data indicate that reduced V-ATPase assembly contributes to the loss of lysosome acidification with age, which affects replicative lifespan.

Structure of yeast RAVE bound to a partial V 1 complex.

Vacuolar-type ATPases (V-ATPases) are membrane-embedded proton pumps that acidify intracellular compartments in almost all eukaryotic cells. Homologous with ATP synthases, these multi-subunit enzymes consist of a soluble catalytic V 1 subcomplex and a membrane-embedded proton-translocating V O subcomplex. The V 1 and V O subcomplexes can undergo reversible dissociation to regulate proton pumping, with reassociation of V 1 and V O requiring the protein complex known as RAVE (regulator of the A TPase of v acuoles and e ndosomes). In the yeast Saccharomyces cerevisiae , RAVE consists of subunits Rav1p, Rav2p, and Skp1p. We used electron cryomicroscopy (cryo-EM) to determine a structure of yeast RAVE bound to V 1 . In the structure, RAVE is a L-shaped complex with Rav2p pointing toward the membrane and Skp1p distant from both the membrane and V 1 . Only Rav1p interacts with V 1 , binding to a region of subunit A not found in the corresponding ATP synthase subunit. When bound to RAVE, V 1 is in a rotational state suitable for binding the free V O complex, but it is partially disrupted in the structure, missing five of its 16 subunits. Other than these missing subunits and the conformation of the inhibitory subunit H, the V 1 complex with RAVE appears poised for reassembly with V O .

Human V-ATPase a-subunit isoforms bind specifically to distinct phosphoinositide phospholipids.

Vacuolar H+-ATPases (V-ATPases) are highly conserved multisubunit enzymes that maintain the distinct pH of eukaryotic organelles. The integral membrane a-subunit is encoded by tissue- and organelle-specific isoforms, and its cytosolic N-terminal domain (aNT) modulates organelle-specific regulation and targeting of V-ATPases. Organelle membranes have specific phosphatidylinositol phosphate (PIP) lipid enrichment linked to maintenance of organelle pH. In yeast, the aNT domains of the two a-subunit isoforms bind PIP lipids enriched in the organelle membranes where they reside; these interactions affect activity and regulatory properties of the V-ATPases containing each isoform. Humans have four a-subunit isoforms, and we hypothesize that the aNT domains of these isoforms will also bind to specific PIP lipids. The a1 and a2 isoforms of human V-ATPase a-subunits are localized to endolysosomes and Golgi, respectively. We determined that bacterially expressed Hua1NT and Hua2NT bind specifically to endolysosomal PIP lipids PI(3)P and PI(3,5)P2 and Golgi enriched PI(4)P, respectively. Despite the lack of canonical PIP-binding sites, we identified potential binding sites in the HuaNT domains by sequence comparisons and existing subunit structures and models. We found that mutations at a similar location in the distal loops of both HuaNT isoforms compromise binding to their cognate PIP lipids, suggesting that these loops encode PIP specificity of the a-subunit isoforms. These data suggest a mechanism through which PIP lipid binding could stabilize and activate V-ATPases in distinct organelles.

The cytosolic N-terminal domain of V-ATPase a-subunits is a regulatory hub targeted by multiple signals.

Vacuolar H+-ATPases (V-ATPases) acidify several organelles in all eukaryotic cells and export protons across the plasma membrane in a subset of cell types. V-ATPases are multisubunit enzymes consisting of a peripheral subcomplex, V1, that is exposed to the cytosol and an integral membrane subcomplex, Vo, that contains the proton pore. The Vo a-subunit is the largest membrane subunit and consists of two domains. The N-terminal domain of the a-subunit (aNT) interacts with several V1 and Vo subunits and serves to bridge the V1 and Vo subcomplexes, while the C-terminal domain contains eight transmembrane helices, two of which are directly involved in proton transport. Although there can be multiple isoforms of several V-ATPase subunits, the a-subunit is encoded by the largest number of isoforms in most organisms. For example, the human genome encodes four a-subunit isoforms that exhibit a tissue- and organelle-specific distribution. In the yeast S. cerevisiae, the two a-subunit isoforms, Golgi-enriched Stv1 and vacuolar Vph1, are the only V-ATPase subunit isoforms. Current structural information indicates that a-subunit isoforms adopt a similar backbone structure but sequence variations allow for specific interactions during trafficking and in response to cellular signals. V-ATPases are subject to several types of environmental regulation that serve to tune their activity to their cellular location and environmental demands. The position of the aNT domain in the complex makes it an ideal target for modulating V1-Vo interactions and regulating enzyme activity. The yeast a-subunit isoforms have served as a paradigm for dissecting interactions of regulatory inputs with subunit isoforms. Importantly, structures of yeast V-ATPases containing each a-subunit isoform are available. Chimeric a-subunits combining elements of Stv1NT and Vph1NT have provided insights into how regulatory inputs can be integrated to allow V-ATPases to support cell growth under different stress conditions. Although the function and distribution of the four mammalian a-subunit isoforms present additional complexity, it is clear that the aNT domains of these isoforms are also subject to multiple regulatory interactions. Regulatory mechanisms that target mammalian a-subunit isoforms, and specifically the aNT domains, will be described. Altered V-ATPase function is associated with multiple diseases in humans. The possibility of regulating V-ATPase subpopulations via their isoform-specific regulatory interactions are discussed.

Human V-ATPase a-subunit isoforms bind specifically to distinct phosphoinositide phospholipids.

V-ATPases are highly conserved multi-subunit enzymes that maintain the distinct pH of eukaryotic organelles. The integral membrane a-subunit is encoded by tissue and organelle specific isoforms, and its cytosolic N-terminal domain (aNT) modulates organelle specific regulation and targeting of V-ATPases. Organelle membranes have specific phosphatidylinositol phosphate (PIP) lipid enrichment linked to maintenance of organelle pH. In yeast, the aNT domains of the two a-subunit isoforms bind PIP lipids enriched in the organelle membranes where they reside; these interactions affect activity and regulatory properties of the V-ATPases containing each isoform. Humans have four a-subunit isoforms. We hypothesize that the aNT domains of the human isoforms will also bind to specific PIP lipids. The a1 and a2 isoforms of human V-ATPase a-subunits are localized to endolysosomes and Golgi, respectively. Bacterially expressed Hua1NT and Hua2NT bind specifically to endolysosomal PIP lipids PI(3)P and PI(3,5)P2 and Golgi enriched PI(4)P, respectively. Despite the lack of canonical PIP binding sites, potential binding sites in the HuaNT domains were identified by sequence comparisons and existing subunit structures and models. Mutations at a similar location in the distal loops of both HuaNT isoforms compromise binding to their cognate PIP lipids, suggesting that these loops encode PIP specificity of the a-subunit isoforms. These data also suggest a mechanism through which PIP lipid binding could stabilize and activate V-ATPases in distinct organelles.

Chimeric a-subunit isoforms generate functional yeast V-ATPases with altered regulatory properties in vitro and in vivo.

V-ATPases are highly regulated proton pumps that acidify organelles. The V-ATPase a-subunit is a two-domain protein containing a C-terminal transmembrane domain responsible for proton transport and an N-terminal cytosolic domain (aNT) that is a regulatory hub, integrating environmental inputs to regulate assembly, localization, and V-ATPase activity. The yeast Saccharomyces cerevisiae encodes only two organelle-specific a-isoforms, Stv1 in the Golgi and Vph1 in the vacuole. On the basis of recent structures, we designed chimeric yeast aNTs in which the globular proximal and distal ends are exchanged. The Vph1 proximal-Stv1 distal (VPSD) aNT chimera binds to the glucose-responsive RAVE assembly factor in vitro but exhibits little binding to PI(3,5)P2. The Stv1 proximal-Vph1 distal (SPVD) aNT lacks RAVE binding but binds more tightly to phosphoinositides than Vph1 or Stv1. When attached to the Vph1 C-terminal domain in vivo, both chimeras complement growth defects of a vph1∆ mutant, but only the SPVD chimera exhibits wild-type V-ATPase activity. Cells containing the SPVD chimera adapt more slowly to a poor carbon source than wild-type cells but grow more rapidly than wild-type cells after a shift to alkaline pH. This is the first example of a "redesigned" V-ATPase with altered regulatory properties and adaptation to specific stresses.

A dual action small molecule enhances azoles and overcomes resistance through co-targeting Pdr5 and Vma1.

Fungal infection threatens human health worldwide due to the limited arsenal of antifungals and the rapid emergence of resistance. Epidermal growth factor receptor (EGFR) is demonstrated to mediate epithelial cell endocytosis of the leading human fungal pathogen, Candida albicans. However, whether EGFR inhibitors act on fungal cells remains unknown. Here, we discovered that the specific EGFR inhibitor osimertinib mesylate (OSI) potentiates azole efficacy against diverse fungal pathogens and overcomes azole resistance. Mechanistic investigation revealed a conserved activity of OSI by promoting intracellular fluconazole accumulation via inhibiting Pdr5 and disrupting V-ATPase function via targeting Vma1 at serine 274, eventually leading to inactivation of the global regulator TOR. Evaluation of the in vivo efficacy and toxicity of OSI demonstrated its potential clinical application in impeding fluconazole resistance. Thus, the identification of OSI as a dual action antifungal with co-targeting activity proposes a potentially effective therapeutic strategy to treat life-threatening fungal infection and overcome antifungal resistance.

Adaptive laboratory evolution in S. cerevisiae highlights role of transcription factors in fungal xenobiotic resistance.

In vitro evolution and whole genome analysis were used to comprehensively identify the genetic determinants of chemical resistance in Saccharomyces cerevisiae. Sequence analysis identified many genes contributing to the resistance phenotype as well as numerous amino acids in potential targets that may play a role in compound binding. Our work shows that compound-target pairs can be conserved across multiple species. The set of 25 most frequently mutated genes was enriched for transcription factors, and for almost 25 percent of the compounds, resistance was mediated by one of 100 independently derived, gain-of-function SNVs found in a 170 amino acid domain in the two Zn2C6 transcription factors YRR1 and YRM1 (p < 1 × 10-100). This remarkable enrichment for transcription factors as drug resistance genes highlights their important role in the evolution of antifungal xenobiotic resistance and underscores the challenge to develop antifungal treatments that maintain potency.

Valproate activates the Snf1 kinase in Saccharomyces cerevisiae by decreasing the cytosolic pH.

Valproate (VPA) is a widely used mood stabilizer, but its therapeutic mechanism of action is not understood. This knowledge gap hinders the development of more effective drugs with fewer side effects. Using the yeast model to elucidate the effects of VPA on cellular metabolism, we determined that the drug upregulated expression of genes normally repressed during logarithmic growth on glucose medium and increased levels of activated (phosphorylated) Snf1 kinase, the major metabolic regulator of these genes. VPA also decreased the cytosolic pH (pHc) and reduced glycolytic production of 2/3-phosphoglycerate. ATP levels and mitochondrial membrane potential were increased, and glucose-mediated extracellular acidification decreased in the presence of the drug, as indicated by a smaller glucose-induced shift in pH, suggesting that the major P-type proton pump Pma1 was inhibited. Interestingly, decreasing the pHc by omeprazole-mediated inhibition of Pma1 led to Snf1 activation. We propose a model whereby VPA lowers the pHc causing a decrease in glycolytic flux. In response, Pma1 is inhibited and Snf1 is activated, resulting in increased expression of normally repressed metabolic genes. These findings suggest a central role for pHc in regulating the metabolic program of yeast cells.

RAVE and Rabconnectin-3 Complexes as Signal Dependent Regulators of Organelle Acidification.

The yeast RAVE (Regulator of H+-ATPase of Vacuolar and Endosomal membranes) complex and Rabconnectin-3 complexes of higher eukaryotes regulate acidification of organelles such as lysosomes and endosomes by catalyzing V-ATPase assembly. V-ATPases are highly conserved proton pumps consisting of a peripheral V1 subcomplex that contains the sites of ATP hydrolysis, attached to an integral membrane V o subcomplex that forms the transmembrane proton pore. Reversible disassembly of the V-ATPase is a conserved regulatory mechanism that occurs in response to multiple signals, serving to tune ATPase activity and compartment acidification to changing extracellular conditions. Signals such as glucose deprivation can induce release of V1 from Vo, which inhibits both ATPase activity and proton transport. Reassembly of V1 with Vo restores ATP-driven proton transport, but requires assistance of the RAVE or Rabconnectin-3 complexes. Glucose deprivation triggers V-ATPase disassembly in yeast and is accompanied by binding of RAVE to V1 subcomplexes. Upon glucose readdition, RAVE catalyzes both recruitment of V1 to the vacuolar membrane and its reassembly with Vo. The RAVE complex can be recruited to the vacuolar membrane by glucose in the absence of V1 subunits, indicating that the interaction between RAVE and the Vo membrane domain is glucose-sensitive. Yeast RAVE complexes also distinguish between organelle-specific isoforms of the Vo a-subunit and thus regulate distinct V-ATPase subpopulations. Rabconnectin-3 complexes in higher eukaryotes appear to be functionally equivalent to yeast RAVE. Originally isolated as a two-subunit complex from rat brain, the Rabconnectin-3 complex has regions of homology with yeast RAVE and was shown to interact with V-ATPase subunits and promote endosomal acidification. Current understanding of the structure and function of RAVE and Rabconnectin-3 complexes, their interactions with the V-ATPase, their role in signal-dependent modulation of organelle acidification, and their impact on downstream pathways will be discussed.

Defining steps in RAVE-catalyzed V-ATPase assembly using purified RAVE and V-ATPase subcomplexes.

The vacuolar H+-ATPase (V-ATPase) is a highly conserved proton pump responsible for the acidification of intracellular organelles in virtually all eukaryotic cells. V-ATPases are regulated by the rapid and reversible disassembly of the peripheral V1 domain from the integral membrane Vo domain, accompanied by release of the V1 C subunit from both domains. Efficient reassembly of V-ATPases requires the Regulator of the H+-ATPase of Vacuoles and Endosomes (RAVE) complex in yeast. Although a number of pairwise interactions between RAVE and V-ATPase subunits have been mapped, the low endogenous levels of the RAVE complex and lethality of constitutive RAV1 overexpression have hindered biochemical characterization of the intact RAVE complex. We describe a novel inducible overexpression system that allows purification of native RAVE and RAVE-V1 complexes. Both purified RAVE and RAVE-V1 contain substoichiometric levels of subunit C. RAVE-V1 binds tightly to expressed subunit C in vitro, but binding of subunit C to RAVE alone is weak. Neither RAVE nor RAVE-V1 interacts with the N-terminal domain of Vo subunit Vph1 in vitro. RAVE-V1 complexes, like isolated V1, have no MgATPase activity, suggesting that RAVE cannot reverse V1 inhibition generated by rotation of subunit H and entrapment of MgADP that occur upon disassembly. However, purified RAVE can accelerate reassembly of V1 carrying a mutant subunit H incapable of inhibition with Vo complexes reconstituted into lipid nanodiscs, consistent with its catalytic activity in vivo. These results provide new insights into the possible order of events in V-ATPase reassembly and the roles of the RAVE complex in each event.

Regulation of V-ATPase Activity and Organelle pH by Phosphatidylinositol Phosphate Lipids.

Luminal pH and the distinctive distribution of phosphatidylinositol phosphate (PIP) lipids are central identifying features of organelles in all eukaryotic cells that are also critical for organelle function. V-ATPases are conserved proton pumps that populate and acidify multiple organelles of the secretory and the endocytic pathway. Complete loss of V-ATPase activity causes embryonic lethality in higher animals and conditional lethality in yeast, while partial loss of V-ATPase function is associated with multiple disease states. On the other hand, many cancer cells increase their virulence by upregulating V-ATPase expression and activity. The pH of individual organelles is tightly controlled and essential for function, but the mechanisms for compartment-specific pH regulation are not completely understood. There is substantial evidence indicating that the PIP content of membranes influences organelle pH. We present recent evidence that PIPs interact directly with subunit isoforms of the V-ATPase to dictate localization of V-ATPase subpopulations and participate in their regulation. In yeast cells, which have only one set of organelle-specific V-ATPase subunit isoforms, the Golgi-enriched lipid PI(4)P binds to the cytosolic domain of the Golgi-enriched a-subunit isoform Stv1, and loss of PI(4)P binding results in mislocalization of Stv1-containing V-ATPases from the Golgi to the vacuole/lysosome. In contrast, levels of the vacuole/lysosome-enriched signaling lipid PI(3,5)P2 affect assembly and activity of V-ATPases containing the Vph1 a-subunit isoform. Mutations in the Vph1 isoform that disrupt the lipid interaction increase sensitivity to stress. These studies have decoded "zip codes" for PIP lipids in the cytosolic N-terminal domain of the a-subunit isoforms of the yeast V-ATPase, and similar interactions between PIP lipids and the V-ATPase subunit isoforms are emerging in higher eukaryotes. In addition to direct effects on the V-ATPase, PIP lipids are also likely to affect organelle pH indirectly, through interactions with other membrane transporters. We discuss direct and indirect effects of PIP lipids on organelle pH, and the functional consequences of the interplay between PIP lipid content and organelle pH.

Interaction between the yeast RAVE complex and Vph1-containing Vo sectors is a central glucose-sensitive interaction required for V-ATPase reassembly.

The yeast vacuolar H+-ATPase (V-ATPase) of budding yeast (Saccharomyces cerevisiae) is regulated by reversible disassembly. Disassembly inhibits V-ATPase activity under low-glucose conditions by releasing peripheral V1 subcomplexes from membrane-bound Vo subcomplexes. V-ATPase reassembly and reactivation requires intervention of the conserved regulator of H+-ATPase of vacuoles and endosomes (RAVE) complex, which binds to cytosolic V1 subcomplexes and assists reassembly with integral membrane Vo complexes. Consistent with its role, the RAVE complex itself is reversibly recruited to the vacuolar membrane by glucose, but the requirements for its recruitment are not understood. We demonstrate here that RAVE recruitment to the membrane does not require an interaction with V1 Glucose-dependent RAVE localization to the vacuolar membrane required only intact Vo complexes containing the Vph1 subunit, suggesting that the RAVE-Vo interaction is glucose-dependent. We identified a short conserved sequence in the center of the RAVE subunit Rav1 that is essential for the interaction with Vph1 in vivo and in vitro Mutations in this region resulted in the temperature- and pH-dependent growth phenotype characteristic of ravΔ mutants. However, this region did not account for glucose sensitivity of the Rav1-Vph1 interaction. We quantitated glucose-dependent localization of a GFP-tagged RAVE subunit to the vacuolar membrane in several mutants previously implicated in altering V-ATPase assembly state or glucose-induced assembly. RAVE localization did not correlate with V-ATPase assembly levels reported previously in these mutants, highlighting both the catalytic nature of RAVE's role in V-ATPase assembly and the likelihood of glucose signaling to RAVE independently of V1.

Whole exome sequencing identified ATP6V1C2 as a novel candidate gene for recessive distal renal tubular acidosis.

Distal renal tubular acidosis is a rare renal tubular disorder characterized by hyperchloremic metabolic acidosis and impaired urinary acidification. Mutations in three genes (ATP6V0A4, ATP6V1B1 and SLC4A1) constitute a monogenic causation in 58-70% of familial cases of distal renal tubular acidosis. Recently, mutations in FOXI1 have been identified as an additional cause. Therefore, we hypothesized that further monogenic causes of distal renal tubular acidosis remain to be discovered. Panel sequencing and/or whole exome sequencing was performed in a cohort of 17 families with 19 affected individuals with pediatric onset distal renal tubular acidosis. A causative mutation was detected in one of the three "classical" known distal renal tubular acidosis genes in 10 of 17 families. The seven unsolved families were then subjected to candidate whole exome sequencing analysis. Potential disease causing mutations in three genes were detected: ATP6V1C2, which encodes another kidney specific subunit of the V-type proton ATPase (1 family); WDR72 (2 families), previously implicated in V-ATPase trafficking in cells; and SLC4A2 (1 family), a paralog of the known distal renal tubular acidosis gene SLC4A1. Two of these mutations were assessed for deleteriousness through functional studies. Yeast growth assays for ATP6V1C2 revealed loss-of-function for the patient mutation, strongly supporting ATP6V1C2 as a novel distal renal tubular acidosis gene. Thus, we provided a molecular diagnosis in a known distal renal tubular acidosis gene in 10 of 17 families (59%) with this disease, identified mutations in ATP6V1C2 as a novel human candidate gene, and provided further evidence for phenotypic expansion in WDR72 mutations from amelogenesis imperfecta to distal renal tubular acidosis.

Interaction of the late endo-lysosomal lipid PI(3,5)P2 with the Vph1 isoform of yeast V-ATPase increases its activity and cellular stress tolerance.

The low-level endo-lysosomal signaling lipid, phosphatidylinositol 3,5-bisphosphate (PI(3,5)P2), is required for full assembly and activity of vacuolar H+-ATPases (V-ATPases) containing the vacuolar a-subunit isoform Vph1 in yeast. The cytosolic N-terminal domain of Vph1 is also recruited to membranes in vivo in a PI(3,5)P2-dependent manner, but it is not known if its interaction with PI(3,5)P2 is direct. Here, using biochemical characterization of isolated yeast vacuolar vesicles, we demonstrate that addition of exogenous short-chain PI(3,5)P2 to Vph1-containing vacuolar vesicles activates V-ATPase activity and proton pumping. Modeling of the cytosolic N-terminal domain of Vph1 identified two membrane-oriented sequences that contain clustered basic amino acids. Substitutions in one of these sequences (231KTREYKHK) abolished the PI(3,5)P2-dependent activation of V-ATPase without affecting basal V-ATPase activity. We also observed that vph1 mutants lacking PI(3,5)P2 activation have enlarged vacuoles relative to those in WT cells. These mutants exhibit a significant synthetic growth defect when combined with deletion of Hog1, a kinase important for signaling the transcriptional response to osmotic stress. The results suggest that PI(3,5)P2 interacts directly with Vph1, and that this interaction both activates V-ATPase activity and protects cells from stress.

Some assembly required: Contributions of Tom Stevens' lab to the V-ATPase field.

Tom Stevens' lab has explored the subunit composition and assembly of the yeast V-ATPase for more than 30 years. Early studies helped establish yeast as the predominant model system for study of V-ATPase proton pumps and led to the discovery of protein splicing of the V-ATPase catalytic subunit. The Vma- phenotype, characteristic of loss-of-V-ATPase activity in yeast was key in determining the enzyme's subunit composition via yeast genetics. V-ATPase subunit composition proved to be highly conserved among eukaryotes. Genetic screens for new vma mutants led to identification of a set of dedicated V-ATPase assembly factors and helped unravel the complex pathways for V-ATPase assembly. In later years, exploration of the evolutionary history of several V-ATPase subunits provided new information about the enzyme's structure and function. This review highlights V-ATPase work in the Stevens' lab between 1987 and 2017.

Compensatory Internalization of Pma1 in V-ATPase Mutants in Saccharomyces cerevisiae Requires Calcium- and Glucose-Sensitive Phosphatases.

Loss of V-ATPase activity in organelles, whether through V-ATPase inhibition or V-ATPase (vma) mutations, triggers a compensatory downregulation of the essential plasma membrane proton pump Pma1 in Saccharomyces cerevisiae We have previously determined that the α-arrestin Rim8 and ubiquitin ligase Rsp5 are essential for Pma1 ubiquination and endocytosis in response to loss of V-ATPase activity. Here, we show that Pma1 endocytosis in V-ATPase mutants does not require Rim101 pathway components upstream and downstream of Rim8, indicating that Rim8 is acting independently in Pma1 internalization. We find that two phosphatases, the calcium-responsive phosphatase calcineurin and the glucose-sensitive phosphatase Glc7 (PP1), and one of the Glc7 regulatory subunits Reg1, exhibit negative synthetic genetic interactions with vma mutants, and demonstrate that both phosphatases are essential for ubiquitination and endocytic downregulation of Pma1 in these mutants. Although both acute and chronic loss of V-ATPase activity trigger the internalization of ∼50% of surface Pma1, a comparable reduction in Pma1 expression in a pma1-007 mutant neither compensates for loss of V-ATPase activity nor stops further Pma1 endocytosis. The results indicate that the cell surface level of Pma1 is not directly sensed and that internalized Pma1 may play a role in compensating for loss of V-ATPase-dependent acidification. Taken together, these results provide new insights into cross talk between two major proton pumps central to cellular pH control.