id: Q16864 gene_symbol: ATP6V1F product_type: PROTEIN status: COMPLETE taxon: id: NCBITaxon:9606 label: Homo sapiens description: ATP6V1F encodes the F subunit (~13 kDa; 119 aa, 13,441 Da, historically called the "14-kDa subunit") of the V1 peripheral sector of the vacuolar-type H+-ATPase (V-ATPase). Together with subunit D, subunit F forms the central rotor of V1 that is driven by ATP hydrolysis in the catalytic A3B3 hexamer and transmits rotational energy to the V0 proteolipid c-ring to drive proton translocation across organelle membranes. ATP6V1F is the smallest subunit of V1 and is ubiquitously expressed, reflecting the housekeeping role of V-ATPase in acidifying lysosomes, endosomes, Golgi apparatus, and other organelles. The D-F central rotor assembly serves as the mechanical connection between the ATP-hydrolyzing head and the proton-translocating V0 membrane sector. In some cell types, the V-ATPase is targeted to the plasma membrane for extracellular acidification. The protein interacts directly with V0 d subunit (ATP6V0D1), cementing its position in the central stalk. Two alternatively spliced isoforms exist. alternative_products: - name: '1' id: Q16864-1 - name: '2' id: Q16864-2 sequence_note: VSP_045952 existing_annotations: - term: id: GO:0016020 label: membrane evidence_type: IBA original_reference_id: GO_REF:0000033 qualifier: is_active_in review: summary: Phylogenetic inference placing ATP6V1F as active in membrane context. The V1 F subunit is a peripheral protein on the cytoplasmic face of membranes where V-ATPase is active. action: MARK_AS_OVER_ANNOTATED reason: The generic membrane annotation with is_active_in is subsumed by the more specific lysosomal membrane and other organelle membrane annotations. The IBA annotation is overly broad. - term: id: GO:0030665 label: clathrin-coated vesicle membrane evidence_type: IEA original_reference_id: GO_REF:0000044 qualifier: located_in review: summary: UniProt subcellular location vocabulary mapping from ortholog data. V-ATPase is present on clathrin-coated vesicles for endocytic pathway acidification. action: KEEP_AS_NON_CORE reason: Consistent with V-ATPase biology but non-core relative to lysosomal function. - term: id: GO:0030672 label: synaptic vesicle membrane evidence_type: IEA original_reference_id: GO_REF:0000044 qualifier: located_in review: summary: UniProt subcellular location vocabulary mapping for synaptic vesicle membrane. V-ATPase acidifies synaptic vesicles for neurotransmitter loading. action: KEEP_AS_NON_CORE reason: Non-core for this ubiquitously expressed subunit; neuronal context is secondary to the primary lysosomal function. - term: id: GO:0033180 label: proton-transporting V-type ATPase, V1 domain evidence_type: IEA original_reference_id: GO_REF:0000002 qualifier: part_of review: summary: InterPro-based annotation placing ATP6V1F in the V1 domain. Confirmed by human cryo-EM structural data. action: ACCEPT reason: Subunit F is a defining structural component of the V1 domain central rotor, confirmed by cryo-EM (PMID:33065002) and biochemical data (PMID:18752060). supported_by: - reference_id: PMID:33065002 supporting_text: Vesicular- or vacuolar-type adenosine triphosphatases (V-ATPases) are ATP-driven proton pumps comprised of a cytoplasmic V1 complex for ATP hydrolysis and a membrane-embedded Vo complex for proton transfer. reference_section_type: ABSTRACT - term: id: GO:0034220 label: monoatomic ion transmembrane transport evidence_type: IEA original_reference_id: GO_REF:0000002 qualifier: involved_in review: summary: InterPro-based annotation for monoatomic ion transmembrane transport, which subsumes proton transport. The more specific proton transmembrane transport annotation is more informative. action: MARK_AS_OVER_ANNOTATED reason: The generic monoatomic ion transmembrane transport is subsumed by the more specific proton transmembrane transport annotations. Redundant and less informative. - term: id: GO:0046961 label: proton-transporting ATPase activity, rotational mechanism evidence_type: IEA original_reference_id: GO_REF:0000002 qualifier: enables review: summary: InterPro-based annotation for rotational ATPase activity. The F subunit is part of the central rotor essential for this activity. action: ACCEPT reason: Core molecular function of the V-ATPase; subunit F is an essential structural component of the rotary mechanism. supported_by: - reference_id: PMID:18752060 supporting_text: Energy from this reaction drives the rotation of a central stalk consisting of V1 subunits D and F and this is coupled to rotation of the V0 proteolipid ring made up of c, c′ and c″. reference_section_type: INTRODUCTION - term: id: GO:1902600 label: proton transmembrane transport evidence_type: IEA original_reference_id: GO_REF:0000002 qualifier: involved_in review: summary: InterPro-based annotation for proton transmembrane transport. action: ACCEPT reason: Core biological process of V-ATPase. - term: id: GO:0005515 label: protein binding evidence_type: IPI original_reference_id: PMID:32296183 qualifier: enables review: summary: Generic protein binding from a reference map of the human binary protein interactome. High-throughput; not informative for specific function. action: MARK_AS_OVER_ANNOTATED reason: High-throughput interactome protein binding annotation is uninformative for the specific function of ATP6V1F. - term: id: GO:0005515 label: protein binding evidence_type: IPI original_reference_id: PMID:33961781 qualifier: enables review: summary: Generic protein binding from a dual proteome-scale interactome network. High-throughput; not informative. action: MARK_AS_OVER_ANNOTATED reason: High-throughput interactome data is uninformative for ATP6V1F function. - term: id: GO:0005515 label: protein binding evidence_type: IPI original_reference_id: PMID:35271311 qualifier: enables review: summary: Generic protein binding from the OpenCell endogenous tagging study. High-throughput; not informative. action: MARK_AS_OVER_ANNOTATED reason: High-throughput protein binding annotation is uninformative. - term: id: GO:0015078 label: proton transmembrane transporter activity evidence_type: IEA original_reference_id: GO_REF:0000107 qualifier: contributes_to review: summary: Ensembl ortholog-transfer annotation for proton transmembrane transporter activity. The contributes_to qualifier appropriately acknowledges the whole-complex nature of this activity. action: ACCEPT reason: Core molecular function of V-ATPase; contributes_to qualifier is appropriate for a structural subunit that participates in but does not individually perform the activity. - term: id: GO:0033176 label: proton-transporting V-type ATPase complex evidence_type: IEA original_reference_id: GO_REF:0000107 qualifier: part_of review: summary: Ensembl ortholog-transfer annotation for V-type ATPase complex membership. Consistent with structural evidence. action: ACCEPT reason: Core complex membership. - term: id: GO:0097401 label: synaptic vesicle lumen acidification evidence_type: IEA original_reference_id: GO_REF:0000107 qualifier: involved_in review: summary: Ensembl ortholog-transfer annotation for synaptic vesicle lumen acidification. V-ATPase acidifies synaptic vesicles; F subunit would be present as part of the complex in neurons. action: KEEP_AS_NON_CORE reason: Synaptic vesicle acidification is a non-core context for this ubiquitous subunit; primary function is lysosomal/organellar acidification. - term: id: GO:0000139 label: Golgi membrane evidence_type: NAS original_reference_id: PMID:32001091 qualifier: located_in review: summary: NAS from V-ATPase review. V-ATPase acidifies the Golgi; F subunit is part of the complex. action: ACCEPT reason: Well-established V-ATPase location in Golgi for glycosylation pathway function. - term: id: GO:0005765 label: lysosomal membrane evidence_type: NAS original_reference_id: PMID:32001091 qualifier: located_in review: summary: NAS from V-ATPase review. Lysosomal membrane is the primary functional location. action: ACCEPT reason: Core localization. - term: id: GO:0005886 label: plasma membrane evidence_type: NAS original_reference_id: PMID:32001091 qualifier: located_in review: summary: NAS from V-ATPase review. V-ATPase is targeted to plasma membrane in specialized cell types. action: KEEP_AS_NON_CORE reason: Plasma membrane localization is real in specialized contexts but non-core for this ubiquitous subunit. - term: id: GO:0007035 label: vacuolar acidification evidence_type: NAS original_reference_id: PMID:32001091 qualifier: involved_in review: summary: NAS from V-ATPase review. Core function of V-ATPase. action: ACCEPT reason: Vacuolar acidification is the core biological process. - term: id: GO:0007042 label: lysosomal lumen acidification evidence_type: NAS original_reference_id: PMID:32001091 qualifier: involved_in review: summary: NAS from V-ATPase review. More specific than vacuolar acidification. action: ACCEPT reason: Core function of V-ATPase. - term: id: GO:0007042 label: lysosomal lumen acidification evidence_type: NAS original_reference_id: PMID:33065002 qualifier: involved_in review: summary: NAS from the structural study. Consistent. action: ACCEPT reason: Core function. - term: id: GO:0010008 label: endosome membrane evidence_type: NAS original_reference_id: PMID:32001091 qualifier: located_in review: summary: NAS from V-ATPase review. V-ATPase acidifies endosomes. action: ACCEPT reason: Endosome membrane is an established V-ATPase location. - term: id: GO:0016020 label: membrane evidence_type: IDA original_reference_id: PMID:33065002 qualifier: located_in review: summary: IDA from the cryo-EM study. F subunit is associated with membrane as part of the V-ATPase complex. action: MARK_AS_OVER_ANNOTATED reason: Generic membrane annotation subsumed by more specific lysosomal/Golgi/endosome membrane annotations. - term: id: GO:0033176 label: proton-transporting V-type ATPase complex evidence_type: NAS original_reference_id: PMID:33065002 qualifier: part_of review: summary: NAS from the structural study. Consistent with IDA annotation from PMID:18752060. action: ACCEPT reason: Core complex membership. - term: id: GO:0048388 label: endosomal lumen acidification evidence_type: NAS original_reference_id: PMID:32001091 qualifier: involved_in review: summary: NAS from V-ATPase review. Endosomal lumen acidification is a core function. action: ACCEPT reason: Core V-ATPase function. - term: id: GO:0051452 label: intracellular pH reduction evidence_type: NAS original_reference_id: PMID:32001091 qualifier: involved_in review: summary: NAS from V-ATPase review. Generic term for the acidification function. action: MARK_AS_OVER_ANNOTATED reason: Less specific than the individual lumen acidification terms; redundant and subsumed by more precise annotations. - term: id: GO:0061795 label: Golgi lumen acidification evidence_type: NAS original_reference_id: PMID:32001091 qualifier: involved_in review: summary: NAS from V-ATPase review. Golgi lumen acidification is important for glycosylation. action: ACCEPT reason: Core V-ATPase function in Golgi. - term: id: GO:1902600 label: proton transmembrane transport evidence_type: NAS original_reference_id: PMID:33065002 qualifier: involved_in review: summary: NAS from the structural study. action: ACCEPT reason: Core biological process. - term: id: GO:0000221 label: vacuolar proton-transporting V-type ATPase, V1 domain evidence_type: ISS original_reference_id: GO_REF:0000024 qualifier: part_of review: summary: Ortholog-based annotation for V1 domain membership. Confirmed by cryo-EM structural data. action: ACCEPT reason: Core structural membership of V1 domain confirmed by PMID:33065002. supported_by: - reference_id: PMID:33065002 supporting_text: Vesicular- or vacuolar-type adenosine triphosphatases (V-ATPases) are ATP-driven proton pumps comprised of a cytoplasmic V1 complex for ATP hydrolysis and a membrane-embedded Vo complex for proton transfer. reference_section_type: ABSTRACT - term: id: GO:0042625 label: ATPase-coupled ion transmembrane transporter activity evidence_type: NAS original_reference_id: PMID:8581736 qualifier: enables review: summary: NAS from the original cloning paper (Fujiwara et al. 1995). ATP6V1F is a component of an ATPase-coupled ion transporter complex. action: ACCEPT reason: The ATPase-coupled ion transmembrane transporter activity is an appropriate molecular function annotation for a V-ATPase subunit. supported_by: - reference_id: PMID:8581736 supporting_text: A cDNA encoding the 14-kDa subunit of vacuolar ATPase was cloned from human fetal brain. The sequence was composed of 680 nucleotides containing an open reading frame of 357 nucleotides. reference_section_type: ABSTRACT - term: id: GO:0070062 label: extracellular exosome evidence_type: HDA original_reference_id: PMID:19199708 qualifier: located_in review: summary: High-throughput proteomics detection in parotid gland exosomes. Likely reflects membrane co-purification. action: MARK_AS_OVER_ANNOTATED reason: Exosome detection is likely artifactual; not informative for core function. - term: id: GO:0070062 label: extracellular exosome evidence_type: HDA original_reference_id: PMID:19056867 qualifier: located_in review: summary: High-throughput proteomics detection in urinary exosomes. action: MARK_AS_OVER_ANNOTATED reason: Same reasoning as parotid exosome; likely artifactual. - term: id: GO:0005829 label: cytosol evidence_type: TAS original_reference_id: Reactome:R-HSA-1222516 qualifier: located_in review: summary: Reactome TAS annotation. V1 F subunit can be in cytosol during regulated V1-V0 disassembly. action: KEEP_AS_NON_CORE reason: The V1 domain including F subunit can exist as a soluble complex in cytosol. - term: id: GO:0005829 label: cytosol evidence_type: TAS original_reference_id: Reactome:R-HSA-5252133 qualifier: located_in review: summary: Reactome TAS annotation for cytosol. action: KEEP_AS_NON_CORE reason: Consistent. - term: id: GO:0005829 label: cytosol evidence_type: TAS original_reference_id: Reactome:R-HSA-74723 qualifier: located_in review: summary: Reactome TAS annotation for cytosol. action: KEEP_AS_NON_CORE reason: Consistent. - term: id: GO:0005829 label: cytosol evidence_type: TAS original_reference_id: Reactome:R-HSA-917841 qualifier: located_in review: summary: Reactome TAS annotation for cytosol. action: KEEP_AS_NON_CORE reason: Consistent. - term: id: GO:0005829 label: cytosol evidence_type: TAS original_reference_id: Reactome:R-HSA-9639286 qualifier: located_in review: summary: Reactome TAS annotation for cytosol. action: KEEP_AS_NON_CORE reason: Consistent. - term: id: GO:0005829 label: cytosol evidence_type: TAS original_reference_id: Reactome:R-HSA-9640167 qualifier: located_in review: summary: Reactome TAS annotation for cytosol. action: KEEP_AS_NON_CORE reason: Consistent. - term: id: GO:0005829 label: cytosol evidence_type: TAS original_reference_id: Reactome:R-HSA-9640168 qualifier: located_in review: summary: Reactome TAS annotation for cytosol. action: KEEP_AS_NON_CORE reason: Consistent. - term: id: GO:0005829 label: cytosol evidence_type: TAS original_reference_id: Reactome:R-HSA-9640175 qualifier: located_in review: summary: Reactome TAS annotation for cytosol. action: KEEP_AS_NON_CORE reason: Consistent. - term: id: GO:0005829 label: cytosol evidence_type: TAS original_reference_id: Reactome:R-HSA-9640195 qualifier: located_in review: summary: Reactome TAS annotation for cytosol. action: KEEP_AS_NON_CORE reason: Consistent. - term: id: GO:0005829 label: cytosol evidence_type: TAS original_reference_id: Reactome:R-HSA-9645598 qualifier: located_in review: summary: Reactome TAS annotation for cytosol. action: KEEP_AS_NON_CORE reason: Consistent. - term: id: GO:0005829 label: cytosol evidence_type: TAS original_reference_id: Reactome:R-HSA-9645608 qualifier: located_in review: summary: Reactome TAS annotation for cytosol. action: KEEP_AS_NON_CORE reason: Consistent. - term: id: GO:0005829 label: cytosol evidence_type: TAS original_reference_id: Reactome:R-HSA-9646468 qualifier: located_in review: summary: Reactome TAS annotation for cytosol. action: KEEP_AS_NON_CORE reason: Consistent. - term: id: GO:0005515 label: protein binding evidence_type: IPI original_reference_id: PMID:18752060 qualifier: enables review: summary: The specific interaction underlying this annotation is the F subunit-V0 d subunit (ATP6V0D1) interaction, which is mechanistically central to the rotary pump mechanism. However, generic protein binding is not informative. action: MARK_AS_OVER_ANNOTATED reason: Generic protein binding is uninformative; the specific D-F and F-d interactions are the mechanistically meaningful interaction, but no specific GO term captures this subunit-rotor interaction. supported_by: - reference_id: PMID:18752060 supporting_text: each can pull down the central stalk's D and F subunits from human kidney membrane, and in vitro studies using D and F further showed that the interactions between these proteins and the d subunit is direct. reference_section_type: ABSTRACT - term: id: GO:0016020 label: membrane evidence_type: IDA original_reference_id: PMID:18752060 qualifier: located_in review: summary: IDA from Smith et al. (2008) showing F subunit in membrane preparations. action: MARK_AS_OVER_ANNOTATED reason: Generic membrane is subsumed by more specific lysosomal/Golgi/endosome membrane annotations. - term: id: GO:0016471 label: vacuolar proton-transporting V-type ATPase complex evidence_type: IDA original_reference_id: PMID:18752060 qualifier: part_of review: summary: IDA from Smith et al. (2008) demonstrating F subunit co-purification with the V-ATPase complex. Direct biochemical evidence for complex membership. action: ACCEPT reason: Direct experimental evidence for V-ATPase complex membership. This is the core complex membership annotation. supported_by: - reference_id: PMID:18752060 supporting_text: each can pull down the central stalk's D and F subunits from human kidney membrane, and in vitro studies using D and F further showed that the interactions between these proteins and the d subunit is direct. reference_section_type: ABSTRACT - term: id: GO:0015078 label: proton transmembrane transporter activity evidence_type: NAS original_reference_id: PMID:8581736 qualifier: enables review: summary: NAS from the original cloning paper. action: ACCEPT reason: Appropriate molecular function for a V-ATPase subunit. - term: id: GO:1902600 label: proton transmembrane transport evidence_type: NAS original_reference_id: PMID:8581736 qualifier: involved_in review: summary: NAS from the original cloning paper. action: ACCEPT reason: Core biological process of V-ATPase. supported_by: - reference_id: PMID:8581736 supporting_text: A cDNA encoding the 14-kDa subunit of vacuolar ATPase was cloned from human fetal brain. The sequence was composed of 680 nucleotides containing an open reading frame of 357 nucleotides. reference_section_type: ABSTRACT references: - id: GO_REF:0000002 title: Gene Ontology annotation through association of InterPro records with GO terms findings: [] - id: GO_REF:0000024 title: Manual transfer of experimentally-verified manual GO annotation data to orthologs by curator judgment of sequence similarity findings: [] - id: GO_REF:0000033 title: Annotation inferences using phylogenetic trees findings: [] - id: GO_REF:0000044 title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt findings: [] - id: GO_REF:0000107 title: Automatic transfer of experimentally verified manual GO annotation data to orthologs using Ensembl Compara findings: [] - id: PMID:18752060 title: The d subunit plays a central role in human vacuolar H(+)-ATPases. findings: - statement: Human V-ATPase F subunit directly interacts with d1 and d2 V0 subunits; F and D are pulled down by d1/d2 from human kidney membrane; confirmed direct D-F and F-d interactions; F forms part of the central stalk. - id: PMID:19056867 title: Large-scale proteomics and phosphoproteomics of urinary exosomes. findings: - statement: ATP6V1F detected in urinary exosomes by mass spectrometry. - id: PMID:19199708 title: Proteomic analysis of human parotid gland exosomes by multidimensional protein identification technology (MudPIT). findings: - statement: ATP6V1F detected in parotid gland exosome proteome. - id: PMID:32001091 title: Structure and Roles of V-type ATPases. findings: - statement: Comprehensive review of V-ATPase structure; F subunit is part of central rotor. - id: PMID:32296183 title: A reference map of the human binary protein interactome. findings: - statement: ATP6V1F detected in binary interactome map. - id: PMID:33065002 title: Structures of a Complete Human V-ATPase Reveal Mechanisms of Its Assembly. findings: - statement: Cryo-EM structures of complete human V-ATPase; F subunit resolved as part of the central DF rotor at near-atomic resolution. - id: PMID:33961781 title: Dual proteome-scale networks reveal cell-specific remodeling of the human interactome. findings: - statement: ATP6V1F detected in proteome-scale interactome study. - id: PMID:35271311 title: 'OpenCell: Endogenous tagging for the cartography of human cellular organization.' findings: - statement: ATP6V1F localization mapped by endogenous tagging. - id: PMID:8581736 title: Cloning, sequencing and expression of a novel cDNA encoding human vacuolar ATPase (14-kDa subunit). findings: - statement: Human V-ATPase F subunit (14 kDa) cloned from fetal brain; ubiquitous expression in human tissues; high sequence conservation with insect orthologs. - id: Reactome:R-HSA-1222516 title: Intraphagosomal pH is lowered to 5 by V-ATPase findings: [] - id: Reactome:R-HSA-5252133 title: ATP6AP1 binds V-ATPase findings: [] - id: Reactome:R-HSA-74723 title: Endosome acidification findings: [] - id: Reactome:R-HSA-917841 title: Acidification of Tf:TfR1 containing endosome findings: [] - id: Reactome:R-HSA-9639286 title: RRAGC,D exchanges GTP for GDP findings: [] - id: Reactome:R-HSA-9640167 title: RRAGA,B exchanges GDP for GTP findings: [] - id: Reactome:R-HSA-9640168 title: v-ATPase:Ragulator:RRAGA,B:GTP:RRAGC,D:GDP:SLC38A9:Arginine dissociates yielding v-ATPase:Ragulator:RRAGA,B:GTP:RRAGC,D:GDP and SLC38A9:Arginine findings: [] - id: Reactome:R-HSA-9640175 title: v-ATPase:Ragulator:RagA,B:GDP:RagC,D:GDP binds SLC38A9:Arginine findings: [] - id: Reactome:R-HSA-9640195 title: RRAGA,B hydrolyzes GTP findings: [] - id: Reactome:R-HSA-9645598 title: RRAGC,D hydrolyzes GTP findings: [] - id: Reactome:R-HSA-9645608 title: v-ATPase:Ragulator:RRAGA,B:GTP:RRAGC,D:GDP binds mTORC1 findings: [] - id: Reactome:R-HSA-9646468 title: mTORC1 binds RHEB:GTP findings: [] core_functions: - description: Central rotor component of the V1 sector of the vacuolar-type H+-ATPase. Together with subunit D, forms the DF central stalk that transmits ATP hydrolysis energy from the catalytic A3B3 hexamer to the V0 c-ring, enabling proton translocation across organelle membranes. Essential for lysosomal, endosomal, and Golgi acidification. The smallest V1 subunit (13 kDa; 119 amino acids) and ubiquitously expressed. contributes_to_molecular_function: id: GO:0046961 label: proton-transporting ATPase activity, rotational mechanism directly_involved_in: - id: GO:1902600 label: proton transmembrane transport locations: - id: GO:0005765 label: lysosomal membrane supported_by: - reference_id: PMID:33065002 supporting_text: Vesicular- or vacuolar-type adenosine triphosphatases (V-ATPases) are ATP-driven proton pumps comprised of a cytoplasmic V1 complex for ATP hydrolysis and a membrane-embedded Vo complex for proton transfer. reference_section_type: ABSTRACT - reference_id: PMID:18752060 supporting_text: Energy from this reaction drives the rotation of a central stalk consisting of V1 subunits D and F and this is coupled to rotation of the V0 proteolipid ring made up of c, c′ and c″. reference_section_type: INTRODUCTION suggested_questions: - question: What is the precise structural role of the F subunit in coordinating the DF central rotor with both the A3B3 head and the V0 d subunit at the rotor-stator junction? experts: [] - question: Are there post-translational modifications on subunit F that regulate V-ATPase assembly or activity, particularly in response to nutrient availability? experts: [] - question: Do the two alternatively spliced isoforms of ATP6V1F differ in their incorporation into the V-ATPase complex or in their subcellular targeting? experts: [] suggested_experiments: - hypothesis: The F subunit makes distinct contacts with V0 d subunit at different stages of the rotary catalytic cycle. description: Cryo-EM analysis of V-ATPase in multiple rotational states at higher resolution to define the precise contacts between F subunit and the V0 d subunit at different stages of the catalytic cycle. experiment_type: structural biology - hypothesis: Post-translational modifications on the F subunit regulate V-ATPase assembly or activity. description: Identification of post-translational modifications on the F subunit using quantitative mass spectrometry under varying nutrient conditions to assess regulation of V-ATPase activity. experiment_type: quantitative mass spectrometry - hypothesis: ATP6V1F isoforms differ in V-ATPase incorporation or subcellular function. description: CRISPR-based isoform knockout combined with rescue experiments using individual isoforms to determine whether either isoform has a distinct functional role in V-ATPase biology. experiment_type: CRISPR functional genomics