CCP2

CCp2 (PF3D7_1455800) is a sexual-stage-specific adhesion-domain protein of Plasmodium falciparum that is essential for parasite transmission through the mosquito vector (PMID:15184503, PMID:17950739). It is expressed across gametocyte stages II–V and during early gamete formation, localizing to the plasma membrane of mature gametocytes within the parasitophorous vacuole and being released during gametogenesis to surround complexes of newly emerged micro- and macrogametes, with expression falling sharply after zygote formation (PMID:15184503, PMID:17950739, PMID:16388802). CCp2 does not act in isolation: it assembles into a multi-protein complex containing all six PfCCp proteins through direct binding between distinct adhesion domains, and this assembly is mutually stabilizing, such that loss of any one member — or of the WD40-repeat partner PfWLP1 — reduces CCp2 protein levels without affecting its transcript (PMID:16388802, PMID:19304662, PMID:35923798). Functionally, targeted disruption of CCp2 permits oocyst sporozoite formation but blocks the transition to salivary-gland invasion, defining a mosquito-stage developmental checkpoint (PMID:15184503), and its surface exposure on gametocytes renders it a target for complement-mediated inhibition of exflagellation by domain-specific antibodies (PMID:17950739). In the rodent malaria P. yoelii, the ccp2 locus drives female-gametocyte-specific expression (PMID:30040976). The molecular activity of CCp2 beyond adhesion-mediated complex assembly has not been characterized in the available corpus.

1. 2004 High

   Establishing whether CCp2 has a non-redundant role in the parasite life cycle, the first knockout defined a specific mosquito-stage transmission checkpoint downstream of oocyst sporozoite formation.

   Evidence Targeted gene disruption by homologous recombination with phenotypic analysis of mosquito-stage development in P. falciparum

   PMID:15184503

   Open questions at the time

   • Molecular basis of the salivary-gland invasion block not identified
   • Does not define which molecular interaction of CCp2 is required for the transition
2. 2004 Medium

   Defining when and where CCp2 acts, expression and localization studies placed the protein at the gametocyte plasma membrane within the parasitophorous vacuole and showed its release around emerging gamete complexes during gametogenesis.

   Evidence Transcript/protein expression analysis and immunofluorescence localization in P. falciparum sexual stages

   PMID:15184503 PMID:16388802 PMID:17950739

   Open questions at the time

   • Mechanism of extracellular release during gametogenesis unknown
   • Functional consequence of the surrounding of gamete complexes not established
3. 2006 Medium

   Testing whether CCp proteins act independently, disruption of PfCCp3 was found to eliminate CCp2 protein but not its transcript, providing the first evidence of post-transcriptional co-dependence indicative of complex formation.

   Evidence Gene disruption with Western blot versus RT-PCR comparison of CCp2 protein and mRNA in disruptant lines

   PMID:16388802

   Open questions at the time

   • Does not demonstrate direct physical interaction between CCp2 and CCp3
   • Single lab
4. 2007 High

   Resolving the stage and cell-type distribution of CCp2 and confirming its transmission role, the protein was mapped to the surface of both sexes of gametocytes and macrogametes but not exflagellating microgametes, with knockout again blocking mosquito-stage development.

   Evidence Immunofluorescence, targeted gene disruption, and mosquito infection assays in P. falciparum

   PMID:17950739

   Open questions at the time

   • Why CCp2 is lost from exflagellating microgametes is unexplained
   • Surface anchoring mechanism not defined
5. 2007 Medium

   Probing whether surface-exposed CCp2 is functionally accessible, domain-specific antibodies were shown to reduce exflagellation in a complement-dependent manner, marking CCp2 as a transmission-blocking-relevant surface antigen.

   Evidence Exflagellation assay with domain-specific antibodies in the presence of complement in P. falciparum

   PMID:17950739

   Open questions at the time

   • Inhibition inferred rather than direct binding demonstrated
   • Which specific domain mediates the effect not resolved
6. 2009 High

   Determining the physical architecture behind the co-dependence, reciprocal Co-IP and recombinant-domain binding assays demonstrated that CCp2 engages in direct adhesion-domain interactions within a six-member PfCCp complex whose members mutually stabilize one another.

   Evidence Co-immunoprecipitation from gametocyte lysates, affinity chromatography co-elution with recombinant domains, and five gene-disruptant lines in P. falciparum

   PMID:19304662

   Open questions at the time

   • Stoichiometry and spatial organization of the complex not resolved
   • No structural model of the CCp2 domain interfaces
   • Single lab
7. 2018 Medium

   Extending the stability network beyond the CCp family, knockdown of the WD40-repeat protein PfWLP1 reduced CCp2 levels, identifying an additional partner required to maintain CCp2 within the adhesion complex.

   Evidence Reverse-genetic knockdown with Western blot quantification of CCp2 in P. falciparum gametocytes

   PMID:35923798

   Open questions at the time

   • Whether PfWLP1 binds CCp2 directly not shown
   • Single lab
8. 2018 Medium

   Assessing conservation of expression pattern across species, tagging of the ccp2 locus in P. yoelii confirmed female-gametocyte-specific expression in a rodent malaria parasite.

   Evidence CRISPR/Cas9 knockin of mCherry downstream of ccp2 with fluorescence imaging in P. yoelii

   PMID:30040976

   Open questions at the time

   • Does not test the function of ccp2 in P. yoelii
   • P. falciparum expression is reported in both sexes, so cross-species divergence in sex-specificity is unresolved

• The biochemical activity of CCp2 and the mechanistic reason its loss blocks the sporozoite-to-salivary-gland transition remain unresolved.
• No enzymatic or ligand-binding activity assigned to CCp2 itself
• The downstream event dependent on CCp2/the CCp complex at the salivary-gland step is unknown
• No structural characterization of CCp2 domains