HAP2/GCS1 (PfHAP2, PF3D7_1014200) is the Plasmodium falciparum member of the ancient, pan-eukaryotic HAP2/GCS1 family of gamete fusogens. It is a single-pass type I membrane protein with a large ectodomain folded as a class II fusion protein, structurally homologous to the fusogens of enveloped viruses, a single transmembrane helix (residues 682-700) and a basic, disordered cytoplasmic tail. The ectodomain carries a hydrophobic cd (fusion) loop that is thought to insert into the target bilayer, and a disulfide-stapled domain 3 whose crystal structure has been solved. Expression is restricted to the sexual stages and, within them, to the male lineage: the protein accumulates internally in developing male gametocytes and, after activation in the mosquito midgut, is displayed throughout the plasma membrane of the flagellated microgamete, where it is accessible to antibodies on intact cells. There it performs the terminal step of fertilization. Species specific surface proteins mediate recognition and tight prefusion attachment between microgamete and macrogamete; HAP2 then merges the two lipid bilayers, so gametes lacking it adhere normally but never fuse. Consistent with this, parasites lacking HAP2 grow and form gametocytes and exflagellate normally but produce no zygotes and no mosquito oocysts, and genetic crosses show the defect is male-specific. Expressed in mammalian cells the protein is a bona fide fusogen, driving cell-cell content mixing and syncytium formation. P. falciparum additionally encodes a second, divergent family member, HAP2p (PF3D7_0816300), which is separately essential for male fertility. Because gamete fusion is an obligatory bottleneck in transmission from human to mosquito, HAP2 is a leading transmission-blocking vaccine antigen.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005783 endoplasmic reticulum | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Phylogenetic propagation from the HAP2/GCS1 ancestral node PTN000788722. The donor evidence is genuine: Arabidopsis HAP2 (AT4G11720, F4JP36) carries an experimental ER annotation and also an ER membrane annotation, and Trypanosoma brucei Q389Q2 has high-throughput ER localization. For PfHAP2 there is no Plasmodium-specific ER evidence, but an internal, pre-activation pool is consistent with the observation that the protein is expressed internally in male stage V gametocytes before being displayed on the microgamete flagellar surface, and a single-pass type I membrane protein with a signal peptide necessarily transits the ER. The claim is therefore plausible and is retained, but the ER is a biosynthetic waystation, not where the fusogen does its work: HAP2 merges bilayers at the microgamete plasma membrane. Reason: The IBD node placement is sound - HAP2/GCS1 is an ancestral single-pass secretory-pathway protein and PfHAP2 sits squarely inside the clade, with no evidence of divergence or loss, so there is no basis for arguing with the node. Retained rather than removed. It is marked non-core rather than accepted as core because the protein's defining activity, membrane merger during gamete fusion, is executed at the plasma membrane of the microgamete and not in the ER; no refinement is proposed because the ER is already as precise as the (indirect) evidence allows for this gene. Propagation Review Root cause: NO FAILURE NON CORE Sources checked: PANTHER:PTN000788722 SUPPORTS TRANSFER HAP2/GCS1 ancestral node. The node is seeded by experimental ER localization of Arabidopsis HAP2 and is biologically coherent for a conserved single-pass type I membrane protein of the secretory pathway; PfHAP2 descends from it with no evidence of divergence. Supporting Evidence: PMID:36241929 This revealed that PfHAP2 was expressed internally in male stage V gametocytes while in the microgametes, it was expressed throughout the flagellum (Fig. 2A). file:PLAF7/HAP2/HAP2-uniprot.txt Belongs to the HAP2/GCS1 family. |
| GO:0005886 plasma membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Mapped from the UniProt subcellular location "Cell membrane; Single-pass type I membrane protein". This is independently confirmed experimentally for PfHAP2: GFP-tagged protein is displayed throughout the flagellum of the activated microgamete, and anti-HAP2 cd loop antibodies stain the surface of non-permeabilized male gametes with the staining lost in HAP2 knockout parasites. The plasma membrane is where the fusogen acts, since bilayer merger with the macrogamete occurs there. Reason: Correct and core. The term is at the right level of granularity: the microgamete is a small flagellated cell whose surface is a single continuous plasma membrane, so no plasma-membrane subregion term would be better supported. GO:0036126 sperm flagellum was considered and rejected as an animal-specific cellular component term inappropriate for a protist microgamete. Supporting Evidence: PMID:36241929 This revealed that PfHAP2 was expressed internally in male stage V gametocytes while in the microgametes, it was expressed throughout the flagellum (Fig. 2A). PMID:29212032 Staining was absent in assays with HAP2-KO parasite lines, indicating the ability of these antibodies to recognize natively folded HAP2 on the gamete surface (Figures 2A and 2B). |
| GO:0140522 fusogenic activity | IDA PMID:36241929 Malaria parasites utilize two essential plasma membrane fuso... | NEW | Summary: GOA gives this protein no molecular function at all, yet PfHAP2 is one of the few characterised non-viral fusogens. Codon-optimized full-length PfHAP2 expressed in BHK cells drives cytoplasmic content mixing and multinucleation to levels comparable to Arabidopsis HAP2, in a heterologous system with no other parasite protein present. Structural work on the family shows a class II viral fusion protein fold with a cd fusion loop whose disruption abolishes gamete fusion, and dye-transfer experiments place the family's requirement precisely at bilayer merger rather than at recognition or attachment. GO:0140522, the activity of joining two lipid bilayers to form a single membrane, is the matching term. Reason: Participation test: the protein itself performs the step. Expressed alone in mammalian cells it fuses them, so it is not merely required for, or a substrate of, another protein's activity; and in gametes, hap2 mutants still recognise and tightly attach to the partner gamete yet fail to merge membranes, isolating HAP2's contribution to the merger step itself. Comparator check: Chlamydomonas HAP2 (A4GRC6) carries experimental annotations for lipid binding and for membrane fusion involvement rather than for GO:0140522, and the GO:0140522 branch is young and sparsely populated; its existing children (SNAP receptor activity, GTPase-dependent fusogenic activity) specify mechanisms PfHAP2 does not use, so the parent is the correct level. The absence of any molecular function on this gene reflects the fact that Q8IJQ3 is an unreviewed TrEMBL entry with only two electronic/phylogenetic annotations, not a curatorial convention against annotating fusogens. Supporting Evidence: PMID:36241929 We found that both PfHAP2 and PfHAP2p induced cytoplasmic content mixing (i.e., cell fusion) to similar levels as the AtHAP2 (Fig. 5A–C). PMID:36241929 both PfHAP2 and PfHAP2p possess independent fusogenic activity as their expression mediated cell fusion, cell content mixing, and syncytia formation. PMID:18367645 Membrane dye experiments show that HAP2 is essential for membrane merger. |
| GO:0009566 fertilization | IMP PMID:36241929 Malaria parasites utilize two essential plasma membrane fuso... | NEW | Summary: Targeted deletion of PF3D7_1014200 in P. falciparum NF54 leaves asexual replication, gametocyte development and exflagellation unaffected but abolishes zygote formation and yields no mosquito oocysts; genetic crosses with female-only (Pfcdpk4-null) and male-only (Pfmacfet-null) producer lines show that the sterility is male-specific, with knockout females fertilised normally. The protein therefore acts in the fusion of the microgamete with the macrogamete, the defining event of fertilization. Reason: Participation test: HAP2 performs the membrane merger that constitutes gamete fusion, so this is participation and not mere necessity - the same protein's fusogenic activity is annotated above, and the knockout phenotype is downstream of that activity rather than independent evidence of a different role. Comparator check: the two best-characterised unicellular HAP2 orthologs in the identical role both carry fertilization terms in GOA - P. berghei HAP2 (Q4YCF6) has GO:0009566 by IMP from PMID:21209845 and PMID:29212032, and Chlamydomonas HAP2 (A4GRC6) carries the gamete-fusion term by IMP - so the absence here is a gap on an unreviewed entry rather than a convention. The taxon-neutral GO:0009566 is proposed rather than its descendant GO:0007342 fusion of sperm to egg plasma membrane involved in single fertilization, because GO:0007342 is classified under GO:0022412 cellular process involved in reproduction in multicellular organism, which does not hold for a protist; that mismatch is raised in suggested_questions instead. Supporting Evidence: PMID:36241929 These results indicate that both PfHAP2 and PfHAP2p are individually essential for zygote formation. PMID:36241929 These results show that Pfhap2¯ males are sterile but females are fertile. PMID:36241929 There were no significant differences observed in the number of exflagellation centers of Pfhap2¯ and Pfhap2p¯ and WT PfNF54 control (Fig. 3C). |
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Download this section (compressed HTML)Q: Should GO provide a taxon-neutral child of GO:0009566 fertilization for gamete membrane fusion in unicellular and anisogamous protists? GO:0007342 fusion of sperm to egg plasma membrane involved in single fertilization is currently used for Chlamydomonas and P. berghei HAP2, yet it is classified under GO:0022412 cellular process involved in reproduction in multicellular organism, which is taxonomically wrong for these organisms, while GO:0000755 cytogamy sits under conjugation with cellular fusion and describes mating type conjugation rather than fusion of differentiated gametes.
Suggested experts: William J Snell, Pascale Gaudet
Q: Is gamete fusion in Plasmodium unidirectional, with HAP2 required only on the male, or bidirectional? Genetic crosses show the requirement is male-restricted, whereas in the BHK heterologous assay fusion occurs only when the fusogen is present in both membranes, implying an unidentified female-side partner.
Suggested experts: Stefan H I Kappe, Benjamin Podbilewicz
Q: What is the functional relationship between PfHAP2 and the paralog PfHAP2p (PF3D7_0816300)? Each is individually essential for male fertility despite both being present in the same microgamete, which is unexpected for two fusogens of the same family; do they form a heterocomplex, or act at successive steps?
Suggested experts: Stefan H I Kappe, Benjamin Podbilewicz
Q: Does PfHAP2 have a defined intracellular residence before activation, and is its delivery to the microgamete surface regulated? The protein is detected internally in stage V male gametocytes, and the only cellular component annotation beyond the plasma membrane is a phylogenetically inferred endoplasmic reticulum term with no Plasmodium data behind it.
Suggested experts: Stefan H I Kappe, Wai-Hong Tham
Experiment: Express and refold the PfHAP2 ectodomain, reconstitute it into proteoliposomes, and measure lipid mixing and content mixing against target liposomes of defined composition using dual fluorescent probes. Include a cd loop mutant and the domain 3 nanobodies as negative controls. This separates the intrinsic fusion activity of the protein from any contribution of host-cell factors present in the BHK assay.
Hypothesis: Purified PfHAP2 ectodomain is sufficient to fuse protein-free liposomes, establishing fusogenic activity for the P. falciparum protein directly rather than through heterologous cell expression.
Type: in vitro liposome fusion assay with recombinant ectodomain
Experiment: Perform reciprocal co-immunoprecipitation and crosslinking mass spectrometry from activated microgametes of the PfHAP2-GFP x PfHAP2p-mCherry double transgenic line, and test whether each protein still reaches the flagellar surface in the other's knockout by immunofluorescence on non-permeabilized gametes.
Hypothesis: PfHAP2 and PfHAP2p are present in the same complex on the microgamete surface, which explains why each is individually essential.
Type: co-immunoprecipitation and surface localization in knockout backgrounds
Experiment: Use the PfHAP2 ectodomain as bait in pulldowns from activated macrogamete surface biotinylation lysates, and test candidate hits by targeted deletion for a female-restricted fertilization defect that phenocopies the male Pfhap2 knockout in genetic crosses.
Hypothesis: A macrogamete surface protein acts as the receptor that makes HAP2-driven fusion unidirectional in the parasite.
Type: affinity purification mass spectrometry plus reverse genetics
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