Plasmodium sporozoites possess both positively and negatively charged sites accessible on their surface

Studies were conducted on the distribution of anionic and cationic sites on the surface of intact Plasmodium berghei and P. yoelii sporozoites. Anionic and cationic ferritins were used as probes for electron microscope studies and fluorescein conjugates of the same charged ferritins were used for correlative studies by fluorescence light microscopy. We found that the surfaces of mature sporozoites from mosquito salivary glands possess both negatively and positively charged sites that are accessible to charged ferritin molecules. Sporozoites obtained from oocysts expressed negatively charged sites but were inconsistent in their expression of positively charged sites. The charged nature of the ferritin probes was confirmed by their binding to cellulose particles or cellular components with known negative or positive charges. The electrostatic nature of the binding of ferritin to sporozoites was established by showing that binding could be blocked by competition with high concentrations of NaCl. The most likely source of these surface charges is circumsporozoite (CS) protein, the major protein covering the surface of sporozoites. Circumsporozoite proteins contain a cluster of positively charged amino acids, which may play a role as components of a surface ligand capable of binding to a liver receptor.