ATP6V1H Research Notes

Gene overview

ATP6V1H (UniProt Q9UI12) encodes V-type proton ATPase subunit H (483 amino acids, ~57 kDa), the regulatory subunit H of the V1 peripheral complex of the vacuolar H+-ATPase (V-ATPase). The protein is also known as NBP1 (Nef-binding protein 1) and VMA13 homolog.

There are two isoforms: isoform 1 (canonical, 483 AA) and isoform 2 (Q9UI12-2, differs at residues 176-193 due to VSP_012274).

V-ATPase complex structure

The V-ATPase is a multi-subunit complex split into the cytoplasmic V1 domain (ATP hydrolysis) and the membrane-embedded V0 domain (proton translocation).

PMID:9442887

PMID:33065002

Role of subunit H specifically

Subunit H is a regulatory subunit of V1, present as one copy. It is essential for V-ATPase activity but not assembly in yeast (VMA13). It is important for regulating the coupling of ATP hydrolysis to proton transport.

UniProt states: "Regulatory subunit of the V1 complex of vacuolar(H+)-ATPase (V-ATPase). The H subunit activates ATPase activity of the V1 subcomplex when dissociated from V0. It has been proposed to inhibit the non-productive hydrolysis of ATP when V1 is in a free state in the cytoplasm."

The H subunit (VMA13/Subunit H) has a unique regulatory role: in the free V1 complex (dissociated from V0), H inhibits futile ATP hydrolysis. When reassociated with V0, it activates the pump. This regulatory switch is important for the regulated disassembly mechanism.

Interaction with AP-2 and endocytic machinery

A key non-V-ATPase function: subunit H (identified as NBP1) binds directly to AP-2 (specifically the medium chain mu2, AP2M1), connecting the V-ATPase to the clathrin-mediated endocytic machinery.

PMID:12032142

PMID:12032142

The biological significance of the V1H-AP2M1 interaction is established: it is not merely a pathogen-hijacking mechanism. The interaction occurs under normal cellular conditions and is part of the endocytic function of the V-ATPase complex on clathrin-coated vesicles.

HIV Nef interaction

Subunit H (NBP1) was originally identified as the binding partner for HIV-1 Nef.

PMID:9620685

PMID:11179428

The Nef-H interaction hijacks the normal V1H-AP2 endocytic function to force CD4 internalization. This is a consequence of H's role as an adaptor connecting V-ATPase to AP-2.

Subcellular localization

Subunit H is detected:
- At lysosomal membrane (HDA, PMID:17897319) — part of assembled V-ATPase
- In cytosol (IDA, GO_REF:0000052 immunofluorescence) — free V1 complex
- At Golgi, endosome, plasma membrane (NAS, PMID:32001091) — assembled V-ATPase at various membranes
- At clathrin-coated vesicle membrane (IEA, UniProt mapping) — consistent with AP-2 interaction

Endocytosis annotation

GO:0006897 endocytosis (IDA, PMID:12032142) — the evidence is that V1H binds AP2M1 and Nef-V1H chimeras can internalize CD4. This demonstrates that V1H contributes to endocytosis via AP-2 interaction. This is a legitimate specific annotation beyond generic "protein binding."

Regulation of macroautophagy (NAS, PMID:22982048)

As for ATP6V1G1, this annotation is an over-annotation. The cited paper uses V-ATPase disruption to impair lysosomal activity; it does not show that H subunit specifically regulates macroautophagy.

Reactome annotations: Nef pathway

Multiple Reactome entries refer to the HIV Nef / CD4 internalization pathway via V-ATPase subunit H. These are placed at the cytosol because V1H bridges cytosolic Nef to AP-2. The localization to cytosol in these entries reflects the V1H function in this endocytic context, not primary V-ATPase localization.

Vacuolar acidification / proton transport annotations

Multiple NAS annotations to vacuolar acidification, endosomal lumen acidification, Golgi lumen acidification, intracellular pH reduction, and proton transmembrane transport from PMID:32001091 and PMID:9442887. These are general review-based annotations for V-ATPase function and are appropriate as NAS.

Summary

ATP6V1H is both a regulatory subunit of the V1 complex (directly modulating ATPase activity in free V1 versus assembled holoenzyme contexts) and a structural adaptor connecting the V-ATPase complex to the AP-2 endocytic machinery via armadillo repeat-mediated AP2M1 binding. The endocytic adaptor function is a legitimate core function of subunit H, distinct from generic V-ATPase proton pumping, and is exploited by HIV/SIV Nef.

Falcon deep research synthesis (2026-06-21)

Falcon deep research has now completed (file:human/ATP6V1H/ATP6V1H-deep-research-falcon.md,
21 citations). It corroborates the regulatory-subunit-H core above and adds
assembly-regulation and disease detail; no change to annotation calls.

• Core confirmed / sharpened. H is a single-copy V1 subunit at the V1–V0
  interface, forming the bottom collar with subunit C and the V0 a-subunit
  N-terminal domain, and contacting the EG peripheral stalk. It couples V1 ATP
  hydrolysis to V0 proton pumping and stabilizes rotational state 1;
  structural/regulatory, not catalytic. Consistent with the known inhibition of
  futile ATP hydrolysis by free V1 (H-dependent). No change to calls.
• Assembly regulation (proteostasis-relevant). V-ATPase assembly involving H
  is modulated by the DMXL1/RAVE complex and by ATG16L1, and by reversible
  V1–V0 disassembly — regulatory inputs for the PN regulation branch.
• TFEB–ATP6V1H axis (Wang 2024). TFEB-dependent Atp6v1h expression is required
  for lysosomal acidification and microglial activation; its disruption worsens tau
  pathology — positioning H in the TFEB–V-ATPase–lysosome feedback loop (non-core,
  disease context).
• Bone/metabolic disease. Atp6v1h deficiency reduces V-ATPase function and
  causes bone loss via a Fos-Jun-Src-Integrin/osteoclast pathway (Zhao 2024);
  reduced ATP6V1H expression is associated with type-2-diabetes progression
  (Molina 2011). Non-core disease context. (H is also NBP1/Nef-binding protein.)

Net: no change to calls — H is the single-copy regulatory V1 subunit at the V1–V0
collar, coupling ATP hydrolysis to proton pumping and gating V-ATPase assembly.