LNPEP (leucyl-cystinyl aminopeptidase; also called insulin-regulated aminopeptidase IRAP, oxytocinase or placental leucine aminopeptidase) is a zinc-dependent M1 metalloaminopeptidase. It is a single-pass type II membrane protein with a short cytosolic N-terminal tail carrying dileucine sorting motifs and a large lumenal/extracellular catalytic domain. It resides in insulin-responsive intracellular vesicles together with GLUT4 and at the plasma membrane, and removes N-terminal residues from peptide hormones and neuropeptides including oxytocin, vasopressin and angiotensin III; it is also the angiotensin IV (AT4) binding site. The donkey protein (1026 aa, full length) falls in the LNPEP subfamily of the M1 family (PANTHER PTHR11533:SF42) with human, mouse and rat LNPEP, is 91% identical to human LNPEP over its whole length, and conserves the zinc ligands, proton acceptor, GAMEN substrate motif, tyrosine transition-state stabilizer and both dileucine sorting motifs. Its functions are inferred from the characterized human and rodent orthologs; no equine experimental data are available.
Summary: Aminopeptidase activity is correct but entailed by the accepted metalloaminopeptidase activity.
Reason: The M1 catalytic machinery is conserved (zinc ligands and proton acceptor), and human LNPEP is a characterized aminopeptidase; metalloaminopeptidase activity is the informative term.
Summary: Cytoplasm is true only in the sense that LNPEP sits on cytoplasmic vesicles; it is entailed by the cytoplasmic vesicle (membrane) localization.
Reason: LNPEP is a type II membrane protein of the plasma membrane and GLUT4-containing intracellular vesicles, not a soluble cytoplasmic protein. The informative intracellular location is cytoplasmic vesicle membrane.
Summary: Plasma membrane localization is established for human LNPEP and expected for the conserved type II donkey protein.
Reason: Human LNPEP is a single-pass type II cell-membrane protein (PMID:11389728 via UniProt) that also cycles from intracellular vesicles to the cell surface; the donkey protein conserves the transmembrane anchor and dileucine sorting motifs.
Summary: Proteolysis is true of an active M1 aminopeptidase but uninformative; the substrates are short peptide hormones and neuropeptides, so peptide catabolic process is the informative process.
Reason: The catalytic residues are conserved and the human ortholog (same PANTHER subfamily PTHR11533:SF42) hydrolyzes oxytocin, vasopressin, angiotensin III, Met-enkephalin and dynorphin. Peptide catabolic process is not an is_a descendant of proteolysis, so this is a lateral refinement to the term the gene already carries (GO:0043171, accepted below), matching the human LNPEP review, which makes the same proteolysis -> peptide catabolic process change.
Summary: Regulation of blood pressure reflects LNPEP degradation of vasoactive peptides (vasopressin, angiotensin III).
Reason: Human LNPEP degrades vasopressin and angiotensin III, providing a physiological link to blood-pressure regulation; this is a downstream organismal process rather than the core activity.
Summary: Zinc ion binding reflects the catalytic zinc of the M1 active site.
Reason: Donkey LNPEP conserves the HExxH-(18)-E zinc ligands (H464, H468, E487). Zinc binding is a cofactor property that underlies, but is not itself, the core activity.
Summary: Cytoplasmic vesicle localization is correct for the GLUT4-associated intracellular pool; for a transmembrane protein the membrane term is more accurate.
Reason: Human LNPEP resides in insulin-responsive intracellular vesicles together with GLUT4 and translocates to the cell surface (shown directly in human adipocytes, PMID:11701721). Two descendants are supported and both are listed. As a type II integral membrane protein it is in the vesicle membrane, so cytoplasmic vesicle membrane applies (as in the human LNPEP review). The specific vesicle is the GLUT4/IRAP/VAMP2 insulin-responsive compartment (GO:0032593, is_a cytoplasmic vesicle via secretory vesicle), which the human review adds as NEW from the same adipocyte data. Transfer to donkey rests on placement in the same PANTHER subfamily (PTHR11533:SF42) and conservation of both N-terminal dileucine sorting motifs that direct the protein to this compartment. No equine localization data exist.
Summary: Peptide binding reflects substrate and angiotensin IV (AT4 ligand) binding.
Reason: Human IRAP/LNPEP binds angiotensin IV and LVV-hemorphin 7 with AT4 receptor characteristics (PMID:11707427). Binding of peptides is part of its enzymatic mechanism and of its AT4-site role; retained as non-core because the catalytic activity is the defining function.
Summary: Peptide catabolic process describes LNPEP degradation of peptide hormones and neuropeptides.
Reason: Human LNPEP hydrolyzes oxytocin, vasopressin and angiotensin III (PMID:1731608) and cleaves Met-enkephalin and dynorphin (PMID:11389728); donkey LNPEP sits in the same PANTHER subfamily (PTHR11533:SF42) and retains all catalytic residues. The substrate range spans peptide hormones (angiotensin III) as well as neuropeptides, so peptide catabolic process is as precise as the existing rows warrant; the neuropeptide catabolic process child (GO:0010813, which PAINT gives human LNPEP by IBA) is recorded in core_functions rather than as a separate NEW row, since it is a descendant of this accepted term.
Summary: Metalloaminopeptidase activity is the core molecular activity.
Reason: Donkey LNPEP is 91% identical to human LNPEP with all M1 catalytic features conserved (zinc ligands H464/H468/E487, proton acceptor E465, GAMEN motif, Tyr transition-state stabilizer); the human enzyme is a characterized zinc aminopeptidase (EC 3.4.11.3).
the enzyme hydrolyzed oxytocin, vasopressin, and angiotensin III
file:EQUAS/LNPEP/LNPEP-bioinformatics/RESULTS.md
ACT_SITE (Proton acceptor): human [465] E -> donkey [465] E.
file:EQUAS/LNPEP/LNPEP-bioinformatics/RESULTS.md
BINDING (): human [464] H -> donkey [464] H.
file:EQUAS/LNPEP/LNPEP-bioinformatics/RESULTS.md
The donkey sequence A0A9L0JGC2 (1026 residues) aligns to human Q9UIQ6 (1025 residues) with 91.02% identity across 1025 paired residues.
Core Functions
Zinc M1 metalloaminopeptidase on the plasma membrane and insulin-responsive vesicle membranes that removes N-terminal residues from peptide hormones and neuropeptides (oxytocin, vasopressin, angiotensin III), terminating their activity.
These computational predictions are reviewed separately from the GOA annotation set used for this review. The assessments below are from this project and do not constitute official GO annotations or endorsement by GO/UniProt. They are not included in the existing annotation review above.
Assessment of 2 ProtNLM2 GO predictions for donkey LNPEP (A0A9L0JGC2), a type II membrane M1 zinc aminopeptidase that is 91% identical to human LNPEP over its full length, with the zinc ligands and proton acceptor conserved. Both predictions are correct but generic; the donkey GOA record already contains each exact term together with a more specific annotation (plasma membrane; metalloaminopeptidase activity), so both are scored LSP. The donkey40 snapshot has no non-GO ProtNLM2 output (function text or location) for this accession.
LSP — Less precise than existing annotation Review score: 2/2
Prediction method: ProtNLM2 · Version: UniProt API snapshot 2026-10-14
Review rationale: The donkey protein has a single N-terminal transmembrane segment (110-131) and human LNPEP is a single-pass type II cell-membrane protein that also resides in GLUT4-containing intracellular vesicles. Membrane is therefore correct. Donkey GOA already contains membrane and the more specific plasma membrane, so the prediction is correct but less precise than an existing annotation.
LSP — Less precise than existing annotation Review score: 2/2
Prediction method: ProtNLM2 · Version: UniProt API snapshot 2026-10-14
Review rationale: Donkey LNPEP conserves the M1 zinc ligands (H464, H468, E487) and the proton acceptor E465, and the human ortholog is a characterized zinc aminopeptidase that hydrolyzes oxytocin, vasopressin and angiotensin III. Metallopeptidase activity is therefore correct. Donkey GOA already contains this exact term (InterPro2GO) and its more specific descendant metalloaminopeptidase activity (TreeGrafter), so the prediction is less precise than an existing annotation.