The subcellular localization of Plasmodium berghei circumsporozoite protein and thrombospondin-related adhesive protein (PbCTRP) in the invasive stage ookinete of P. berghei was studied in the midgut of Anopheles stephensi by immuno-electron microscopic observations using polyclonal antibodies and immuno-gold labeling. PbCTRP was found to be associated with the micronemes of a mature ookinete throughout the movement from the endoperitrophic space to the basal lamina of the midgut epithelium. PbCTRP was also observed in the electron-dense area outside the ookinete, which might have been secreted from the apical pore. PbCTRP is found most abundantly at the site of contact between the apical end of an ookinete and the basal lamina of an epithelial cell. These results suggest that PbCTRP functions as an adhesion molecule for ookinete movement into the midgut lumen and epithelial cell and for ookinete association with the midgut basal lamina and transformation into an oocyst.

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