Conserved regions from Plasmodium falciparum MSP11 specifically interact with host cells and have a potential role during merozoite invasion of red blood cells.

Despite significant global efforts, a completely effective vaccine against Plasmodium falciparum, the species responsible for the most serious form of malaria, has not been yet obtained. One of the most promising approaches consists in combining chemically synthesized minimal subunits of parasite proteins involved in host cell invasion, which has led to the identification of peptides with high binding activity (named HABPs) to hepatocyte and red blood cell (RBC) surface receptors in a large number of sporozoite and merozoite proteins, respectively. Among these proteins is the merozoite surface protein 11 (MSP11), which shares important structural and immunological features with the antimalarial vaccine candidates MSP1, MSP3, and MSP6. In this study, 20-mer-long synthetic peptides spanning the complete sequence of MSP11 were assessed for their ability to bind specifically to RBCs. Two HABPs with high ability to inhibit invasion of RBCs in vitro were identified (namely HABPs 33595 and 33606). HABP-RBC bindings were characterized by means of saturation assays and Hill analysis, finding cooperative interactions of high affinity for both HABPs (n(H) of 1.5 and 1.2, K(d) of 800 and 600 nM for HABPs 33595 and 33606, respectively). The nature of the possible RBC receptors for MSP11 HABPs was studied in binding assays to enzyme-treated RBCs and cross-linking assays, finding that both HABPs use mainly a sialic acid-dependent receptor. An analysis of the immunological, structural and polymorphic characteristics of MSP11 HABPs supports including these peptides in further studies with the aim of designing a fully effective protection-inducing vaccine against malaria.

Conserved high activity binding peptides are involved in adhesion of two detergent-resistant membrane-associated merozoite proteins to red blood cells during invasion.

Detergent resistant membranes (DRMs) of Plasmodium falciparum merozoites contain a large number of glycosylphosphatidylinositol (GPI)-anchored proteins that have been implicated in interactions between merozoites and red blood cells (RBCs). In this study, two cysteine-rich proteins anchored by GPI to merozoite DRMs (Pf92 and Pf113) were studied with the aim of identifying regions actively involved in RBC invasion. By means of binding assays, high-activity binding peptides (HABPs) with a large number of binding sites per RBC were identified in Pf92 and Pf113. The nature of the RBC surface receptors for these HABPs was explored using enzyme-treated RBCs and cross-linking assays. Invasion inhibition and immunofluorescence localization studies suggest that Pf92 and Pf113 are involved in RBC invasion and that their adhesion to RBCs is mediated by such HABPs. Additionally, polymorphism and circular dichroism studies support their inclusion in further studies to design components of an antimalarial vaccine.