Structure of an immunodominant epitope of the circumsporozoite surface protein of Plasmodium knowlesi.

Previous studies have shown that the immunodominant region of the circumsporozoite surface (CS) protein of Plasmodium knowlesi is contained within a tandemly repeated dodecapeptide: Gln-Ala-Gln-Gly-Asp-Gly-Ala-Asn-Ala-Gly-Gln-Pro. We show here that the CS protein epitopes reacting with six monoclonal antibodies raised against the intact parasite are represented in a synthetic tandem repeat of this dodecapeptide. The specificity of four of these antibodies was studied further by preparing synthetic peptides corresponding to overlapping regions of the repeats and measuring their ability to inhibit the specific interaction between the antibodies and CS proteins. We find that three antibodies have very similar patterns of reactivity with this series of peptides and that they define an epitope of eight amino acids (Gly-Asp-Gly-Ala-Asn-Ala-Gly-Gln) within the dodecapeptide. The remaining antibody probably recognizes a configurational epitope formed by a tandem repeat of the dodecapeptide.