Deep research status: Falcon deep research has now completed successfully (ATP6AP1-deep-research-falcon.md, 19 citations); see the synthesis section at the end of this file. The original PN-batch attempt timed out before the deep_research_unified tool bugs were fixed. This review uses that report together with the fetched UniProt and GOA records, cached publications, local Reactome entries, and the PN projection report.
Core function synthesis: ATP6AP1/Ac45 is best reviewed as an accessory/regulatory V0-sector subunit of the V-ATPase that supports complex assembly, targeting, stability, and activity rather than as a catalytic proton pump subunit. Human V-ATPase structure work defines ATP6AP1 as a V0 assembly hub PMID:33065002. The human deficiency study supports an Ac45/Voa1-like assembly-factor role and shows disease mutants fail to restore V-ATPase-dependent growth in yeast PMID:27231034.
Localization/function context: Ac45 localizes mainly to the ER and ERGIC in hepatocytes, not predominantly to the TGN or endosomal system in that experiment PMID:27231034. Mature V-ATPase activity is nevertheless central to endolysosomal and secretory-compartment acidification, and Reactome represents ATP6AP1 as an accessory V0 subunit facilitating acidification [Reactome:R-HSA-5252133 "V-type proton ATPase subunit S1 (ATP6AP1) is thought to function as an accessory subunit of the V0 subcomplex of V-ATPase, facilitating acidification"].
PN projection decision: the PN projection reports GO:0007042 lysosomal lumen acidification as already present in GOA and projects ATP6AP1 to GO:0060590 ATPase regulator activity [file:projects/PROTEOSTASIS/reports/pn_projection/pn_projected_annotations.tsv "ATPase regulator activity is the narrowest GO target that preserves the source mechanism without requiring a speculative complex-specific term."]. I accepted the lysosomal acidification projection as already supported and added GO:0060590 conservatively as a new reviewed MF term, using the same evidence to modify the broad GO:0140677 molecular function activator activity row.
Conservative exclusions: generic GO:0005515 protein binding rows were marked over-annotated because they do not describe the ATP6AP1 molecular function. Direct GO:0031267 small GTPase binding rows were removed because the key mTORC1 paper supports V-ATPase-dependent Rag nucleotide loading but did not detect direct Rag interactions with purified V-ATPase subunits PMID:22053050. HIF/iron and broad trafficking terms were kept as non-core secondary consequences where supported PMID:28296633.
The Falcon report (file:human/ATP6AP1/ATP6AP1-deep-research-falcon.md) is fully
consistent with the assembly-factor / structural-hub framing above and adds
structural and pathway detail. Citations use the report's DOIs except where a
PMID is already established in this file.
Structural confirmation of the V0 hub role (Wang et al. 2020, Mol Cell 80:501,
doi:10.1016/j.molcel.2020.09.029; the "structural hub" finding already cited here
as PMID:33065002). The cryo-EM structure shows ATP6AP1's luminal domain adopts a
β-prism fold homologous to LAMP-family domains (despite no sequence homology),
sits centrally inside the c-ring, and is the most connected V0 subunit
(>7,000 Ų buried surface), contacting ATP6AP2, c'', c(1/2/8/9) and subunit d.
This directly supports the assembly/stability MF over any catalytic role and
explains, at residue level, why disease mutations are loss-of-function: Y313C
destabilizes the luminal hydrophobic core, E346K disrupts a luminal salt bridge,
M428I perturbs TM contacts with the c-ring.
Cooperative V0 assembly with ATP6AP2 (Guida et al. 2018,
doi:10.1091/mbc.e18-04-0234). ATP6AP1 and ATP6AP2/(pro)renin receptor together
act as ER V0 assembly factors replacing the single yeast Voa1 function;
co-expression rescues V-ATPase-defective yeast and restores vacuolar quinacrine
accumulation. Relevant context for the neighboring ATP6AP2 review.
Autophagy linkage (non-core, recent/translational). Fei et al. 2024
(doi:10.1016/... breast-cancer lysosomal-gene study) and Yan et al. 2025 report
that ATP6AP1 supports autophagic flux via lysosomal acidification and, newly,
promotes autophagosome-lysosome fusion by enhancing Rab7-HOPS interaction;
overexpression drives chemoresistance. These are downstream/disease-context roles
and should remain non-core relative to the V0 assembly MF, but the Rab7-HOPS
fusion link is worth flagging as a candidate ALP-branch secondary function if it
is independently corroborated.
Other corroborated points (no change to calls): ER assembly then trafficking
to endolysosomes, Golgi (glycosylation), neuroendocrine secretory vesicles, and
osteoclast ruffled border; tissue-specific proteolytic processing (~40 kDa brain,
62 kDa liver, ~50 kDa B cells); V-ATPase-dependent (structural, not pump-direct)
role in mTORC1 amino-acid sensing. Net: no change to the core call (accessory V0
assembly/structural-hub subunit; GO:0060590 ATPase regulator activity + lysosomal
acidification), with strengthened structural justification.