A pleckstrin homology domain protein is involved in crystalloid formation in <i>Plasmodium</i> ookinetes and affects the maturation of infective sporozoites

The <i>Plasmodium</i> ookinete is a unique invasive stage which is formed following the fertilization of gametes in mosquito midguts after ingestion of infected mammalian blood. Unlike merozoites and sporozoites, ookinetes do not differentiate and proliferate inside cells; and rather penetrate the midgut epithelium and migrate through to the basal lamina, where they transform into oocysts. Sporozoites develop inside oocysts, and because sporozoites ultimately initiate malarial infections to humans, the prior ookinete stage is correspondingly a promising target to reduce malaria transmission. Ookinetes possess a distinctive crystalloid organelle which is named for its crystalline-like array of spherical particles. Recent research indicates that crystalloids influence the formation of oocysts and subsequent sporozoites, but the molecular mechanisms of the process are not understood. Here we focused on a novel crystalloid protein in <i>Plasmodium yoelii</i> that contains a pleckstrin homology (PH) domain. The protein is a paralog of the previously identified CryPH, and therefore we designated it PyCryPH2. Targeted disruption of the <i>pycryph2</i> gene resulted in an irregular microstructure of particles within the crystalloids. While <i>pycryph2</i>-deficient parasites developed into morphologically normal sporozoites, the parasites had a significantly reduced ability to invade mosquito salivary glands, as well as to infect mouse liver. These findings indicate that PyCryPH2 is involved in forming crystalloids with a well-organized internal architecture and contributes to the maturation of functional sporozoites either directly or through interaction with other crystalloid proteins.