HAP2 (PfHAP2 / PfGCS1, PF3D7_1014200, UniProt Q8IJQ3) notes

Automated deep research was unavailable for this gene (Falcon authentication failed and the
OpenAI key was invalid). These notes are a manual literature review: the UniProt record, the
GOA file, and primary papers fetched into publications/ and read directly. No
-deep-research-*.md file was created.

Identity

• UniProt Q8IJQ3 (TrEMBL, unreviewed), "Male gamete fusion factor HAP2, putative", 889 aa,
  chromosome 10, ORF PF3D7_1014200. The UniProt entry carries no gene symbol, so the review keeps
  gene_symbol: Q8IJQ3; the folder uses the community name HAP2.
• HAP2/GCS1 family (InterPro IPR040326; PANTHER PTHR31764 "PROTEIN HAPLESS 2", subfamily SF0).
• Topology (UniProt features): signal peptide, large ectodomain, single helical transmembrane
  segment at 682-700, cytoplasmic tail 701-889 that is largely disordered and basic. A crystal
  structure of domain 3 (residues 500-620; PDB 9YC3) shows disulfides 515-525 and 548-594.
• Plasmodium has two HAP2/GCS1 genes: PfHAP2 (this gene) and the paralog PfHAP2p
  (PF3D7_0816300) PMID:36241929.
  Care is needed not to conflate the two in annotation.

Family-level biology

• HAP2 is the ancestral eukaryotic gamete fusogen and a class II fusion protein homologous to
  viral fusogens PMID:28235200.
• In Chlamydomonas and P. berghei, hap2 mutant gametes still attach but cannot merge membranes
  PMID:18367645;
  PMID:18367645.
• P. berghei GCS1 knockout gives male sterility PMID:18403203.

P. falciparum-specific evidence

Kumar et al. 2022 (PMID:36241929) is the key primary paper on this gene.

• Expression/localization: male-specific; internal in stage V male gametocytes, then throughout
  the flagellum of the microgamete PMID:36241929;
  not seen in females PMID:36241929.
• Knockout: no asexual or gametocyte or exflagellation defect, but no zygotes and no oocysts
  PMID:36241929.
• Genetic crosses: male-restricted function PMID:36241929.
• Fusogenic activity: expressed in BHK cells, PfHAP2 drives cell-cell fusion to a level similar
  to Arabidopsis HAP2, and fusion needs the protein in both cells in that heterologous system
  PMID:36241929;
  PMID:36241929.
  The authors propose a female-side partner may make fusion unidirectional in vivo; unresolved.
• Transmission-blocking target: anti-cd (fusion) loop antibodies against PfHAP2 reduced
  P. falciparum transmission in field membrane feeds PMID:29212032 (note: line break in cached text; quote in YAML uses
  a single-line substring). Recombinant PfHAP2 elicits IgG with strong SMFA activity
  PMID:24042109.
• Surface exposure: HAP2 is accessible to antibodies on non-permeabilized male gametes (shown in
  P. berghei) PMID:29212032.
• Structure: domain 3 crystal structure with nanobodies PMID:41498187.

Annotation thinking

• GO:0005886 plasma membrane (IEA, SubCell): correct; the fusogen acts on the microgamete
  plasma membrane. ACCEPT.
• GO:0005783 endoplasmic reticulum (IBA, PTN000788722; donors Arabidopsis HAP2 F4JP36 with IDA
  ER, and T. brucei Q389Q2 with HTP/IDA ER): plausible as a pre-activation/biosynthetic pool (PfHAP2
  is "expressed internally" in gametocytes before activation), but not where the protein acts. No
  Plasmodium ER data. KEEP_AS_NON_CORE.
• Missing: no MF and no BP at all in GOA. Proposed NEW:
• GO:0140522 fusogenic activity (IDA, PMID:36241929, BHK cell fusion). Participation test is
  trivially passed: HAP2 itself merges the bilayers.
• GO:0007342 fusion of sperm to egg plasma membrane involved in single fertilization (IMP,
  PMID:36241929). Comparator check (QuickGO): P. berghei HAP2 Q4YCF6 carries GO:0007342 (IMP, PMID:21209845, PMID:29212032) and GO:0009566 fertilization (IMP, same
  papers), and Chlamydomonas HAP2 A4GRC6 carries GO:0007342 (IMP, PMID:25655701). So both the
  unicellular comparators in the same role carry the fertilization terms; the absence on PfHAP2
  is a curation gap for an unreviewed TrEMBL entry, not a convention.
• Caveat: GO:0007342 is a child of GO:0022412 "cellular process involved in reproduction in
  multicellular organism", which is a poor taxonomic fit for a protist even though GO curators
  have used it for Chlamydomonas and P. berghei HAP2. For this review I propose the
  taxon-neutral GO:0009566 fertilization (also used for P. berghei HAP2) and raise the
  GO:0007342 taxon mismatch as a question for experts rather than asserting it here.
• GO:0000755 cytogamy (fusion of the cytoplasm of two mating cells of a single-celled organism)
  was considered as the taxon-appropriate cell-fusion term. It sits under GO:0000747 conjugation
  with cellular fusion, which describes mating-type conjugation rather than anisogamous
  gamete fusion, so it is not a good fit and I did not propose it.
• Protist-specific checks performed: no plant/animal-only terms are present in GOA (both existing
  rows are generic compartments); no apicoplast, rhoptry, microneme, parasitophorous vacuole or
  digestive vacuole terms are involved; no host-vs-symbiont process terms (HAP2 acts on the
  parasite's own gametes inside the mosquito midgut, not on host cells); the one paralog risk
  (PfHAP2p, PF3D7_0816300) was checked and the evidence cited is explicitly PfHAP2-specific.