Identification of a PH domain-containing protein which is localized to crystalloid bodies of Plasmodium ookinetes

For the success of the malaria control and eradication programme it is essential to reduce parasite transmission by mosquito vectors. In the midguts of mosquitoes fed with parasite-infected blood, sexual-stage parasites fertilize to develop into motile ookinetes that traverse midgut epithelial cells and reside adjacent the basal lamina. Therefore, the ookinete is a promising target of transmission-blocking vaccines to break the parasite lifecycle in mosquito vectors. However, the molecular mechanisms of ookinete formation and invasion of epithelial cells have not been fully elucidated. A unique structure called the crystalloid body has been identified in the ookinete cytoplasm by electron microscopy, but its biological functions remain unclear.

CryPH, containing a signal peptide and PH domain, is predominantly expressed in zygotes and ookinetes and is localized to crystalloid bodies in P. yoelii. CryPH accumulates in vesicle-like structures prior to the appearance of typical crystalloid bodies. Unlike other known crystalloid body localized proteins, CryPH does not appear to have a multiple domain architecture characteristic of the LAP/CCp family proteins. Although CryPH is highly conserved among Plasmodium, Babesia, Theileria, and Cryptosporidium, PyCryPH is dispensable for the development of invasive ookinetes and sporozoites in mosquito bodies.

A recombinant protein of a novel molecule, designated as crystalloid body specific PH domain-containing protein of Plasmodium yoelii (PyCryPH), was synthesized using a wheat germ cell-free system. Specific rabbit antibodies against PyCryPH were obtained to characterize the expression and localization of PyCryPH during sexual-stage parasite development. In addition, PyCryPH knockout parasites were generated by targeted gene disruption to examine PyCryPH function in mosquito-stage parasite development.

Western blot and immunofluorescence assays using specific antibodies showed that PyCryPH is specifically expressed in zygotes and ookinetes. By immunoelectron microscopy it was demonstrated that PyCryPH is localized within crystalloid bodies. Parasites with a disrupted PyCryPH gene developed normally into ookinetes and formed oocysts on the basal lamina of midguts. In addition, the number of sporozoites residing in salivary glands was comparable to that of wild-type parasites. Conclusions: CryPH, containing a signal peptide and PH domain, is predominantly expressed in zygotes and ookinetes and is localized to crystalloid bodies in P. yoelii. CryPH accumulates in vesicle-like structures prior to the appearance of typical crystalloid bodies. Unlike other known crystalloid body localized proteins, CryPH does not appear to have a multiple domain architecture characteristic of the LAP/CCp family proteins. Although CryPH is highly conserved among Plasmodium, Babesia, Theileria, and Cryptosporidium, PyCryPH is dispensable for the development of invasive ookinetes and sporozoites in mosquito bodies.