P25

P25 (PF3D7_1031000) is a GPI-anchored surface protein expressed on zygotes and ookinetes that protects the parasite during its developmental passage through the mosquito midgut and promotes its transformation into oocysts (PMID:11483501, PMID:10989152). In P. berghei, single knockout of P25 causes only mild defects, whereas simultaneous loss of P25 and its paralog P28 severely impairs ookinete survival against midgut lethal factors including proteases, epithelial traversal, and oocyst formation, establishing that the two proteins act redundantly in these functions (PMID:11483501). P25 mediates parasite–vector interactions through direct binding to mosquito midgut ligands: in P. berghei it engages Anopheles gambiae laminin gamma 1 (PMID:11223130), and in P. vivax it binds calreticulin on the apical surface of Anopheles albimanus midgut microvilli (PMID:17442413). P25 can homodimerize and form heterodimers with P28 (PMID:10984433), and loss of both proteins abolishes the SRPN10 serpin 'time bomb' response in invaded midgut epithelial cells (PMID:15659062). Structurally, the P. vivax ortholog Pvs25 adopts a triangular prism built from four EGF-like domains whose triangle-forming residues are conserved across P25/P28 orthologs, consistent with surface tiling (PMID:16327807); the eleven disulfide bonds of P. falciparum Pfs25 have been mapped by mass spectrometry and match this architecture (PMID:29162427). As a leading transmission-blocking vaccine target, P. falciparum P25 presents at least two non-overlapping immunogenic sites whose antibodies additively inhibit parasites (PMID:29146922). Expression of P. falciparum P25 depends on defined 3′ cis-regulatory elements within ~315 nt of the stop codon (PMID:16439031).

1. 2000 Medium

   Establishing where P25 resides and how it oligomerizes was needed to define it as a surface effector; biochemistry showed GPI anchoring and yeast two-hybrid revealed homo- and heterodimerization with P28, linking the two paralogs physically.

   Evidence GPI anchor biochemical analysis and yeast two-hybrid dimerization assays with in vitro ookinete co-culture in P. berghei

   PMID:10984433 PMID:10989152

   Open questions at the time

   • Dimerization shown only by yeast two-hybrid, not confirmed on the native ookinete surface
   • Functional consequence of dimerization not dissected
2. 2001 High

   Whether P25 has a genetically defined role in mosquito-stage development, and whether it acts alone, was resolved by single and double knockouts showing partial redundancy with P28 in protecting ookinetes and enabling midgut traversal and oocyst formation.

   Evidence Single and double gene knockout in P. berghei with mosquito infection and ookinete/oocyst phenotyping

   PMID:11483501

   Open questions at the time

   • Molecular basis of protection against midgut proteases not defined
   • Individual contribution of P25 versus P28 not separable due to redundancy
3. 2001 Medium

   To assign a receptor/ligand role, P25 was tested against mosquito proteins, revealing a specific interaction with Anopheles gambiae laminin gamma 1 stronger than that of P28.

   Evidence Yeast two-hybrid and in vitro binding of purified fusion proteins in P. berghei

   PMID:11223130

   Open questions at the time

   • In vivo relevance of the laminin interaction to midgut traversal not demonstrated
   • Binding interface not mapped
4. 2005 Medium

   The link between P25/P28 surface proteins and the host midgut defensive response was addressed by showing double-knockout ookinetes fail to trigger SRPN10 serpin redistribution, connecting the parasite surface to epithelial 'time bomb' signaling.

   Evidence Double KO P25/P28 parasites with immunofluorescence and quantitation of SRPN10 in P. berghei midgut cells

   PMID:15659062

   Open questions at the time

   • Response attributed jointly to P25 and P28, not to P25 alone
   • Mechanism linking surface protein to serpin induction unknown
5. 2005 High

   The three-dimensional architecture underlying P25 function and antibody recognition was defined by the Pvs25 crystal structure, revealing a four-EGF triangular prism with conserved triangle residues and a heavy-chain-only antibody contact at a vertex.

   Evidence X-ray crystallography of Pvs25 alone and cocrystal with a transmission-blocking antibody in P. vivax

   PMID:16327807

   Open questions at the time

   • Proposed surface tiling not directly observed on ookinetes
   • Structure is of the P. vivax ortholog, not PF3D7_1031000 itself
6. 2006 Medium

   How P25 expression is controlled at the mRNA level was mapped, identifying 3′ cis elements within ~315 nt of the stop codon, a required T-rich region, and an enhancing nonamer.

   Evidence Transient transfection deletion constructs and 3′ RACE in P. falciparum

   PMID:16439031

   Open questions at the time

   • Trans-acting factors binding these elements not identified
   • Stage-specificity of regulation not established
7. 2007 High

   A second midgut ligand for P25 was identified, showing Pvs25 specifically binds calreticulin on Anopheles albimanus microvillar surfaces, distinguishing it from P28.

   Evidence Blot overlay, LC-MS identification, immunofluorescence, cDNA cloning, and co-IP of recombinant proteins in P. vivax

   PMID:17442413

   Open questions at the time

   • Functional role of calreticulin binding in traversal or oocyst formation not tested
   • Interaction demonstrated for the P. vivax ortholog
8. 2010 Low

   Whether P25 is retained or shed during motility was addressed by detecting the protein on gliding trails, indicating it is deposited onto substrates during ookinete locomotion.

   Evidence Protein detection on gliding trails of P. berghei ookinetes on solid substrates

   PMID:19747017

   Open questions at the time

   • Single lab, single method with limited mechanistic follow-up
   • Whether shedding is functionally required for motility not tested
9. 2017 High

   For P. falciparum P25 as a vaccine antigen, the number and location of protective epitopes were defined, revealing two non-overlapping immunogenic sites whose antibodies additively inhibit transmission, and confirming the correct disulfide architecture of recombinant Pfs25.

   Evidence Six crystal structures of Pfs25–antibody complexes with transmission-blocking assays, and LC-MS/MS mapping of eleven disulfide bonds in P. falciparum

   PMID:29146922 PMID:29162427

   Open questions at the time

   • Native surface disposition inferred rather than directly imaged
   • Relationship of epitope sites to endogenous mosquito-ligand binding not defined

• The precise molecular mechanism by which P25 binding to laminin gamma 1 and calreticulin drives midgut traversal, protease protection, and oocyst transformation—and how these functions are partitioned from the redundant paralog P28—remains unresolved.
• No structure of P25 bound to a mosquito ligand
• Individual, non-redundant function of P25 versus P28 not separated
• Causal link between ligand binding and specific developmental phenotypes untested