Apical membrane antigen 1 (AMA1, PF3D7_1133400) is a single-pass type I membrane protein of the apicomplexan AMA1 family, expressed in late schizonts and merozoites of the asexual blood stage of Plasmodium falciparum. It is made as an 83-kDa precursor that is trafficked to the micronemes, where its N-terminal prodomain is removed, and the mature protein is then translocated onto the merozoite surface around the time of schizont rupture. During erythrocyte invasion AMA1 forms the core of the tight (moving) junction: a hydrophobic groove in its ectodomain, uncovered by displacement of the flexible domain II loop, binds a disulfide-bonded loop of RON2, a rhoptry neck protein that the parasite delivers into the erythrocyte membrane together with RON4 and RON5. This pairing marks the merozoite's commitment to invasion and anchors the junction through which the actomyosin motor draws the parasite into a nascent parasitophorous vacuole; conditional depletion of PfAMA1 lowers invasion efficiency and impairs resealing of the erythrocyte membrane behind the parasite. Gene deletions in Plasmodium berghei and Toxoplasma gondii indicate that AMA1 also contributes to attachment of zoites to host cells. The ectodomain is highly polymorphic and is a leading blood-stage malaria vaccine antigen.
Definition: The ring-shaped, electron-dense zone of contact formed between the plasma membrane of an invading apicomplexan zoite and the host cell membrane, containing the parasite surface protein AMA1 and the rhoptry neck protein complex, and through which the parasite moves into a nascent parasitophorous vacuole. Also called the tight junction.
Justification: GO has microneme, rhoptry and apical complex for apicomplexan structures, but a search of the cellular component branch returns no term for the moving (tight) junction, which is the structure where AMA1 actually performs its function and is referred to as a distinct entity throughout the apicomplexan invasion literature. An existing-annotation term cannot be used for it; plasma membrane is the closest available location.
Parent term: cell junction
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005515 protein binding | IPI PMID:16835460 Plasmodium falciparum AMA1 binds a rhoptry neck protein homo... | MODIFY | Summary: Co-immunoprecipitation of PfAMA1 with PfRON4 (PF3D7_1116000), a rhoptry neck protein of the moving-junction RON complex. Reason: The co-immunoprecipitation is sound and shows that AMA1 associates with the RON complex, but the link to RON4 is most likely indirect, via RON2, which is the RON-complex component shown to contact AMA1 directly (PMID:22737069). Bare protein binding conveys no functional information. The RON complex is secreted from the rhoptry necks and inserted into the erythrocyte membrane before AMA1 engages it, so the informative molecular function for this interaction is host cell surface binding, which this gene already carries by IDA. Proposed replacements: host cell surface binding Supporting Evidence: PMID:16835460 Plasmodium falciparum apical membrane antigen 1 (PfAMA1) coimmunoprecipitates with the Plasmodium homologue of TgRON4, a secreted rhoptry neck protein of Toxoplasma gondii that migrates at the moving junction in association with TgAMA1 during invasion. PMID:22737069 In particular, RON2, a transmembrane component of the RON complex, interacts directly with AMA1. |
| GO:0005515 protein binding | IPI PMID:22737069 Structural and functional insights into the malaria parasite... | MODIFY | Summary: Structurally characterized direct binding of the PfAMA1 ectodomain to a peptide from the extracellular region of PfRON2 (PF3D7_1452000), with alanine substitution of key contact residues abolishing binding. Reason: The AMA1-RON2 pairing is the defining molecular interaction of this protein and rests on crystal structures plus mutagenesis, but the term protein binding conveys none of that. Because RON2 is inserted into the erythrocyte membrane before AMA1 binds it, the activity is binding to a ligand displayed at the host cell surface, captured by GO:0046812, which this gene already carries by IDA. GO:0046789 host cell surface receptor binding would be wrong, since RON2 is a parasite protein rather than a host receptor. Proposed replacements: host cell surface binding Supporting Evidence: PMID:22737069 The receptor-binding site of PfAMA1 comprises the hydrophobic groove and a region that becomes exposed by displacement of the flexible Domain II loop. PMID:22737069 Mutations of key contact residues of PfRON2 and PfAMA1 abrogate binding between the recombinant proteins. |
| GO:0016020 membrane | IEA GO_REF:0000120 | MODIFY | Summary: Electronic assignment from the AMA1 family InterPro signature plus the UniProt single-pass membrane subcellular location. Reason: AMA1 is a type I transmembrane protein, so membrane is true but less specific than the evidence supports. After prodomain removal in the micronemes the mature protein is translocated onto the merozoite surface, i.e. the parasite plasma membrane, where it engages RON2 during invasion. Replace with the is_a descendant plasma membrane. Proposed replacements: plasma membrane Supporting Evidence: PMID:12228305 Apical membrane antigen 1 of Plasmodium falciparum (PfAMA1) contains an N-terminal propeptide that is removed prior to the translocation of the mature protein onto the merozoite surface. PMID:2701947 Apical membrane antigen 1 appears to be transported to the merozoite surface near the time of schizont rupture. |
| GO:0020007 apical complex | TAS PMID:2701947 Integral membrane protein located in the apical complex of P... | MODIFY | Summary: The 1989 paper that cloned the antigen and named it for its apparent location at the apical end of the merozoite. Reason: Apical complex is correct but less specific than later evidence supports. Immuno-electron microscopy localized the unprocessed protein to the micronemes, which are part_of the apical complex, and that specific term is already carried by this gene with IDA evidence. Replace with microneme. Proposed replacements: microneme Supporting Evidence: PMID:2701947 This antigen is designated apical membrane antigen 1 because it appears to be located in the apical complex. PMID:12228305 We localized unprocessed PfAMA1 to the microneme organelles of the intraerythrocytic schizont. |
| GO:0020009 microneme | IDA PMID:12228305 Independent translocation of two micronemal proteins in deve... | ACCEPT | Summary: Immunofluorescence and immuno-electron microscopy with a prodomain-specific monoclonal antibody place the unprocessed precursor in the micronemes of intraerythrocytic schizonts. Reason: Direct, organelle-level localization in the correct organism and stage. The microneme is a genuine apicomplexan compartment and is the site where the precursor is stored and N-terminally processed before release to the merozoite surface, so this is a core location for the protein. Supporting Evidence: PMID:12228305 In the present study, we used a MAb targeted to the N-terminal pro-region of PfAMA1 to demonstrate conclusively by immunofluorescence microscopy and immuno-EM that unprocessed AMA1 is located in the micronemes. PMID:12228305 AMA1 is synthesized as an 83-kDa precursor protein that traffics to and is N-terminally processed within the micronemes. |
| GO:0044409 symbiont entry into host | IMP PMID:24571085 Conditional expression of apical membrane antigen 1 in Plasm... | MODIFY | Summary: Conditional (DiCre) excision of Pfama1 in P. falciparum reduces protein levels by about 80% within one cycle, cutting growth by 40% through lower invasion efficiency and a post-invasion defect in sealing the parasitophorous vacuole. Reason: Loss-of-function evidence in the correct organism and stage, and AMA1 performs part of the work of entry rather than merely being required for it: its ectodomain is the parasite-side anchor of the tight junction through which the merozoite draws itself into the erythrocyte. GO:0044409 is the right orientation (the symbiont enters the host) but covers entry into the body, tissues or cells alike, so it is less specific than the evidence supports. The child GO:0046718 symbiont entry into host cell is framed around breaching the membrane and releasing the symbiont or its genome into the cytoplasm, which does not describe apicomplexan invasion either. The correct descendant is GO:0085017 entry into host cell by a symbiont-containing vacuole, whose definition is invasion that forms a vacuole in which the symbiont resides, beginning with attachment to a host membrane that invaginates as the symbiont enters and ending when the host membrane closes behind the new vacuole. That is exactly the moving-junction invasion AMA1 anchors, and the cited evidence turns on the parasitophorous vacuole itself (a post-invasion defect in sealing it). GOA already uses GO:0085017 for P. falciparum merozoite and sporozoite invasion proteins (MSP1 by IDA, CSP by IMP), so this also aligns AMA1 with the term used across the parasite's invasion machinery. Proposed replacements: entry into host cell by a symbiont-containing vacuole Supporting Evidence: PMID:24571085 These results show that PfAMA1 is an essential protein for merozoite invasion in P. falciparum and either directly or indirectly plays a role in resealing of the red blood cell at the posterior end of the invasion event. PMID:21788485 Collectively, these results identify the binding of RON2 to the hydrophobic pocket of AMA1 as the step that commits Plasmodium merozoites to RBC invasion and point to RON2 as a potential vaccine candidate. |
| GO:0046812 host cell surface binding | IDA PMID:24108241 Apical membrane antigen 1 mediates apicomplexan parasite att... | ACCEPT | Summary: Zoites lacking AMA1 are impaired in association with host cells, supporting an adhesive activity of AMA1 directed at the host cell surface; structurally, AMA1 engages RON2 after the parasite has delivered it into the erythrocyte membrane. Reason: The activity is well supported: AMA1 binds a ligand displayed in the host cell membrane, which is exactly what GO:0046812 describes, and the structural work on the AMA1-RON2 complex establishes the binding chemistry in P. falciparum itself. The cited paper's attachment assays were performed with P. berghei merozoites and T. gondii tachyzoites rather than P. falciparum, so the P. falciparum IDA rests partly on the shared mechanism; the curator read the full text and the claim is consistent with the P. falciparum structural and conditional-knockdown data, so it is accepted. Note that GO:0046789 host cell surface receptor binding would be wrong, because the bound partner RON2 is a parasite protein inserted into the host membrane, not a host receptor. Supporting Evidence: PMID:24108241 Therefore, as with the Toxoplasma tachyzoite, AMA1 favours Plasmodium merozoite attachment to, but not internalization into, the host cell. PMID:22737069 The MJ contains two key parasite components: the surface protein Apical Membrane Antigen 1 (AMA1) and its receptor, the Rhoptry Neck Protein (RON) complex, which is targeted to the host cell membrane during invasion. |
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Download this section (compressed HTML)Q: Does P. falciparum AMA1 contribute measurably to merozoite attachment to erythrocytes, as its P. berghei and T. gondii counterparts do, and would a GO annotation to adhesion of symbiont to host cell (GO:0044650) be warranted for the P. falciparum protein?
Q: Why do the P. berghei and T. gondii knockouts tolerate loss of AMA1 for penetration while P. falciparum conditional knockdown impairs invasion? Is there a compensating ligand in the knockout lines, or a species difference in how the junction is anchored?
Q: Is the resealing defect seen on PfAMA1 depletion a direct role for AMA1 at the posterior end of the invading merozoite, or an indirect consequence of a structurally weaker junction?
Q: Should GO add a cellular component term for the apicomplexan moving (tight) junction, so that AMA1 and the RON proteins can be located in the structure where they act rather than only in plasma membrane?
Experiment: Quantitative attachment assays (flow cytometry and live imaging of merozoite to erythrocyte contacts) using the P. falciparum DiCre conditional Pfama1 line, with and without the RON2 peptide, to test directly in P. falciparum the adhesion role established in the rodent and Toxoplasma knockouts.
Hypothesis: PfAMA1-depleted merozoites show reduced initial attachment to erythrocytes independently of their defect in junction formation.
Experiment: Complementation of the P. falciparum conditional knockdown with AMA1 variants carrying hydrophobic-groove mutations that abolish RON2 binding but retain surface trafficking, scoring junction formation, penetration and erythrocyte resealing separately by live imaging and electron microscopy.
Hypothesis: Junction anchoring and the resealing step are separable activities of AMA1, with only the former requiring an intact RON2-binding groove.
Experiment: Correlative light and electron microscopy, or expansion microscopy, of invading merozoites expressing tagged AMA1 and RON2, to map the spatial distribution of AMA1 through junction assembly, translocation and closure and provide the structural basis for a moving junction cellular component term.
Hypothesis: AMA1 is concentrated in the junction ring and is progressively excluded or shed as the junction translocates to the posterior pole.
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