Targeting the histone-fold dimerization interface of oocyst rupture proteins from Plasmodium berghei for antimalarial inhibitor discovery

Dimerization between the histone-fold domains (HFD) of two Plasmodium Oocyst Rupture Proteins (ORP1 and ORP2) is essential for oocyst rupture in the Anopheles mosquito vector host, representing a key event in parasite transmission to humans. Notably, ORPs are a rare example of HFD-containing proteins that operate outside the nucleus and that lack DNA-binding functions, typically associated with core histones and transcription factors hosting deviant histones. ORP HFD heterodimerization occurs at the outer capsule of the oocyst, immediately prior to rupture, thus providing a temporal window to administer dimerization blocking molecules. In this context, we present the first detailed structural analysis of the HFD ORP heterodimer, solved by X-ray crystallography at 3.1 Å resolution, and analyze the oligomerization interface as a possible druggable target. Targeting the mosquito phase of the parasite lifecycle remains an under-exploited avenue as present antimalarial therapies mainly target the human blood stages of infection. We employed a GAL4-based yeast two-hybrid (Y2H) combinatorial library of cyclic peptides (CPs) to identify six candidates that inhibit dimerization in vitro. Molecular docking simulations confirmed that all six CPs bind at the dimer interface, allowing us to rank them for further in vivo testing of their efficacy in blocking oocyst rupture.