CTRP

CTRP (PF3D7_0315200) is a large type I transmembrane micronemal protein expressed specifically in the ookinete stage of Plasmodium, where it is essential for ookinete infectivity of the mosquito: gene disruption in both P. berghei and P. falciparum yields ookinetes that develop normally but fail to invade the midgut epithelium and never form oocysts, abolishing mosquito transmission (PMID:10562534, PMID:10587361, PMID:10760158). A mixed-infection rescue in P. berghei established that CTRP acts cell-autonomously within the ookinete to mediate midgut invasion (PMID:10587361). Structurally, CTRP has a ~1990-residue extracellular region built from six A-type (integrin-like) domain repeats followed by seven thrombospondin type 1 (TSR) repeats, a transmembrane domain, and a short cytoplasmic segment, encoded as a single-copy gene (PMID:8719155). It localizes to micronemes concentrated at the apical/anterior end of the ookinete, confirmed by immunofluorescence, immunogold electron microscopy (peripheral, membrane-associated within the microneme), and micronemal-fraction proteomics (PMID:10760158, PMID:10377190, PMID:15069061, PMID:19206106). Functionally, CTRP is a member of the TRAP invasin family: its cytoplasmic domain binds the actin-bridging molecule aldolase, coupling extracellular adhesion to the intracellular actomyosin motor (PMID:18441124), and its transmembrane domain is cleaved by the rhomboid protease PfROM4 (PMID:17040128). Antisera against CTRP block ookinete infectivity to mosquitoes, marking it as a transmission-blocking target (PMID:15069061).

1. 1995 Medium

   Establishing CTRP's primary structure was the first step in defining it as a candidate adhesin, revealing a modular extracellular architecture suited to receptor engagement.

   Evidence Molecular cloning and sequence/domain analysis of PF3D7_0315200 in P. falciparum

   PMID:8719155

   Open questions at the time

   • No ligand or receptor identified for the A-type or TSR domains
   • Domain functions inferred from sequence, not tested biochemically
2. 1999 High

   The question of whether CTRP has an essential in vivo role was answered by knockout, defining it as required for ookinete motility and midgut invasion.

   Evidence Gene disruption by homologous recombination with motility assays and mosquito infection in P. berghei

   PMID:10562534

   Open questions at the time

   • Molecular mechanism of the motility defect not defined at this stage
   • Adhesion receptor not identified
3. 1999 High

   A mixed-infection rescue experiment resolved whether CTRP acts cell-autonomously, showing its function is intrinsic to the ookinete for midgut invasion.

   Evidence Gene disruption plus heterologous mating/mixed feeding rescue in P. berghei; stage-specific IFA localization

   PMID:10377190 PMID:10587361

   Open questions at the time

   • Does not define the invasion step (adhesion vs. penetration) that fails
   • No partner proteins identified
4. 2000 High

   Confirming CTRP essentiality and apical localization in the human parasite P. falciparum extended the ookinete-invasion role across species and tied it to a TRAP-like micronemal distribution.

   Evidence Homologous recombination disruption, IFA, and membrane feeding assays in P. falciparum

   PMID:10760158

   Open questions at the time

   • Localization by IFA not yet resolved to the micronemal compartment ultrastructurally
   • No mechanistic link to the motor established
5. 2004 Medium

   Ultrastructural mapping and antibody inhibition addressed exactly where CTRP resides within the microneme and whether it is a viable intervention target.

   Evidence Immunogold double-labeling EM and mosquito infectivity inhibition with anti-CTRP antisera in P. gallinaceum

   PMID:15069061

   Open questions at the time

   • Mechanism of antibody-mediated blocking (secretion vs. adhesion) not defined
   • Shown in avian-malaria species; human-parasite relevance inferred
6. 2006 Medium

   Whether CTRP is processed like other TRAP-family invasins was addressed by testing it as a rhomboid substrate, implicating regulated intramembrane cleavage in its function.

   Evidence Heterologous PfROM4 cleavage assay with transmembrane-domain swaps in P. falciparum

   PMID:17040128

   Open questions at the time

   • CTRP-specific mutagenesis of the cleavage site not reported
   • Cleavage not demonstrated in vivo in the ookinete
   • CTRP was one of many substrates tested
7. 2008 Medium

   The mechanistic link between CTRP's extracellular adhesion and the parasite motor was established by showing its cytoplasmic tail engages aldolase, defining it as a functional TRAP-family invasin.

   Evidence Aldolase-binding assay and genetic complementation with the TRAP cytoplasmic domain in P. berghei

   PMID:18441124

   Open questions at the time

   • Direct coupling to myosin motor force in ookinetes not demonstrated
   • Aldolase-binding motif within CTRP not mapped
8. 2009 Medium

   Proteomic confirmation settled CTRP's compartment assignment, placing it among the most abundant known ookinete micronemal proteins.

   Evidence Microneme subcellular fractionation with EM enrichment and gel LC-MS/MS in P. berghei

   PMID:19206106

   Open questions at the time

   • Abundance does not establish functional interactions within the microneme
9. 2012 Low

   Whether CTRP localization is under regulatory control was probed via PPKL loss, placing CTRP downstream of a phosphatase pathway governing ookinete cytoskeleton and invasion.

   Evidence ppkl gene knockout with IFA of CTRP localization in P. berghei

   PMID:23028336

   Open questions at the time

   • Indirect readout within a PPKL-focused study; mislocalization mechanism not resolved
   • Direct CTRP-PPKL relationship not tested
10. 2017 Low

    The dependence of CTRP maturation on TSR glycosylation was addressed indirectly through POFUT2 disruption, implicating O-fucosylation in folding/trafficking of TSR-bearing proteins including CTRP.

    Evidence POFUT2 genetic disruption with trafficking and life-cycle fitness assays in P. falciparum

    PMID:28916755

    Open questions at the time

    • Direct O-fucosylation of CTRP not individually demonstrated
    • CTRP implicated only as a member of the TSR-protein substrate class

• The mosquito midgut receptor(s) engaged by CTRP's A-type and TSR domains, and the direct in vivo consequences of PfROM4 cleavage and O-fucosylation for CTRP function, remain undefined.
• No extracellular ligand/receptor identified
• In vivo relevance of TM cleavage unresolved
• Structure of the CTRP ectodomain not determined