TRAMP

TRAMP (PF3D7_1218000) is a thrombospondin-related apical membrane protein of the Plasmodium merozoite that participates in erythrocyte invasion by tethering the invasion machinery to the parasite surface (PMID:38123677). It is an apical organelle protein of intraerythrocytic schizont-stage merozoites in both P. falciparum and P. berghei that relocates onto the merozoite surface prior to or during invasion (PMID:15003842), and in P. falciparum it is shed from the surface by the serine protease PfSUB2 during erythrocyte entry (PMID:16879884). Its secretion from rhoptries is triggered by adhesion of the microneme protein EBA175 to its erythrocyte receptor glycophorin A, and antibodies against TRAMP block invasion, additively with anti-EBA175 antibodies (PMID:23387921). Structurally, TRAMP forms a complex with PfCSS on the parasite membrane; this PfCSS–PfTRAMP module is engaged by the elongated tail of PfRIPR, thereby linking the PfRCR complex (PfRH5, PfCyRPA, PfRIPR) to the merozoite surface and positioning PfRH5 and PfCyRPA to engage erythrocyte receptors during invasion (PMID:38123677). Reports of TRAMP's direct erythrocyte-binding activity diverge across studies: recombinant TRAMP failed to bind erythrocytes in one assay (PMID:22467743), whereas native and recombinant TRAMP bound treated and untreated erythrocytes in another (PMID:23387921).

1. 2004 Medium

   Establishing where TRAMP resides and when it becomes accessible was the first step in placing it in the invasion pathway; localization to developing apical organelles with subsequent surface relocation marked it as an invasion-stage protein.

   Evidence Immunofluorescence localization in intraerythrocytic schizonts of P. falciparum and P. berghei, co-localizing with AMA-1

   PMID:15003842

   Open questions at the time

   • No functional consequence of localization demonstrated
   • Whether apical residence is micronemal versus rhoptry was not resolved here (later contested)
   • Trigger and mechanism of surface relocation not defined
2. 2006 Medium

   Identifying the protease that removes TRAMP from the merozoite surface connected its dynamics to the regulated proteolytic events of invasion.

   Evidence Biochemical shedding assay with identification of PfSUB2 as the sheddase in P. falciparum

   PMID:16879884

   Open questions at the time

   • Single lab, single study
   • Functional consequence of shedding for invasion not shown
   • Cleavage site and kinetics not defined
3. 2012 Medium

   Testing whether TRAMP is a direct erythrocyte adhesin and a vaccine-relevant invasion target gave negative results, constraining early functional models.

   Evidence Erythrocyte binding assay with recombinant extracellular domain and in vitro growth inhibition assay with TRAMP antibodies

   PMID:22467743

   Open questions at the time

   • Negative binding result conflicts with the 2013 study
   • Recombinant domain may not reproduce native conformation
   • Antibody epitopes and accessibility not characterized
4. 2013 Medium

   Re-examination placed TRAMP in the rhoptry bulb and linked its secretion to EBA175–glycophorin A engagement, arguing for a functional role in invasion supported by antibody inhibition.

   Evidence Immunofluorescence localization, native and recombinant erythrocyte binding assays, and invasion inhibition assays with polyclonal antibodies in P. falciparum

   PMID:23387921

   Open questions at the time

   • Erythrocyte binding conflicts with the 2012 study; the discrepancy is unresolved
   • Rhoptry bulb versus microneme localization diverges from earlier report
   • Direct erythrocyte receptor for TRAMP not identified
5. 2023 High

   Structural determination defined TRAMP's molecular role as an anchoring partner, showing it forms a PfCSS–PfTRAMP complex that captures the PfRIPR tail to tether the PfRCR invasion complex to the merozoite surface.

   Evidence Cryo-EM structure of the PfRCR complex with erythrocyte-binding assay and structural modelling in P. falciparum

   PMID:38123677

   Open questions at the time

   • Whether TRAMP contributes affinity or only scaffolding to receptor engagement not dissected
   • Stoichiometry and dynamics of the PfCSS–PfTRAMP complex during live invasion not defined
   • Relationship of this membrane-anchoring role to the earlier reported EBA175-triggered secretion not integrated

• It remains unresolved whether TRAMP itself directly engages an erythrocyte receptor or acts purely as a surface-anchoring scaffold for the PfRCR complex, and the conflicting erythrocyte-binding data have not been reconciled.
• No defined erythrocyte receptor for TRAMP
• Binding-positive and binding-negative results not reconciled across studies
• No genetic loss-of-function phenotype in the corpus to establish essentiality