ORP2

ORP2 (PF3D7_1439500) is a histone-fold domain (HFD)-containing protein that acts at the Plasmodium oocyst capsule to enable oocyst rupture and sporozoite egress (PMID:27982038). Its N-terminal HFD mediates heterodimerization with the partner protein ORP1 at the capsule at the time of rupture, and this HFD is essential for ORP2 function, since its disruption or loss of either partner blocks oocyst rupture (PMID:27982038). Distinct regions of ORP2 contribute independently to egress: the HFD is required for sporozoite egress, and progressive C-terminal deletions also abolish or severely impair egress (PMID:30391497). The ORP1/ORP2 heterodimer nucleates assembly of a trimeric NF-Y-like complex by recruiting a third subunit, ORP3, through ORP3's conserved NF-YA helix A1, and this trimeric assembly at the oocyst periphery is required for excystation and sporozoite egress (PMID:42045549). The ORP1/ORP2 HFD heterodimer interface has been resolved by X-ray crystallography at 3.1 Å, and cyclic peptides that bind this interface inhibit dimerization in vitro, defining the oligomerization surface as a druggable target (PMID:41506730). All characterization of ORP2 derives from Plasmodium berghei.

1. 2016 High

   Established that ORP2 is a histone-fold protein whose function at the oocyst capsule is required for oocyst rupture, answering what molecular activity underlies its role in the parasite life cycle.

   Evidence Genetic knockout, HFD mutational analysis, and in vitro heterodimer reconstitution with ORP1 in P. berghei

   PMID:27982038

   Open questions at the time

   • Structural basis of the ORP1/ORP2 HFD interface not resolved at this stage
   • How the heterodimer physically drives capsule opening was not defined
   • No identification of additional complex members
2. 2018 High

   Dissected the modular contribution of ORP2 regions, showing that both the N-terminal HFD and C-terminal segments are independently needed for sporozoite egress rather than the HFD alone.

   Evidence Deletion-mutant analysis with oocyst rupture and salivary gland sporozoite counts in P. berghei

   PMID:30391497

   Open questions at the time

   • Molecular partners or activities of the C-terminal regions unidentified
   • Mechanism by which C-terminal deletions impair egress not defined
3. 2026 High

   Resolved the ORP1/ORP2 HFD heterodimer interface at atomic resolution and demonstrated it can be pharmacologically targeted, establishing the oligomerization surface as essential and druggable.

   Evidence X-ray crystallography at 3.1 Å, GAL4-based yeast two-hybrid cyclic-peptide screen, in vitro dimerization inhibition, and molecular docking in P. berghei

   PMID:41506730

   Open questions at the time

   • In vivo efficacy of the inhibitory cyclic peptides not demonstrated
   • Structure of the higher-order (trimeric) assembly not resolved
4. 2026 High

   Extended the ORP2 mechanism from a dimer to a trimeric NF-Y-like complex, showing the ORP1/ORP2 dimer recruits ORP3 to form the functional unit required for excystation.

   Evidence Genetic KO of orp3, yeast-3-hybrid trimerization assay, and ORP3 localization to the oocyst periphery in P. berghei

   PMID:42045549

   Open questions at the time

   • Biochemical mechanism by which the trimer effects capsule opening remains unknown
   • Whether additional subunits or regulators join the complex is undetermined

• The physical mechanism by which the ORP1/ORP2/ORP3 trimeric complex triggers oocyst capsule opening and sporozoite egress remains unresolved.
• No effector activity linking the complex to capsule disassembly identified
• Function of ORP2 in human-infective Plasmodium species not established
• No structure of the full trimeric assembly