Plasmodium falciparum: red blood cell binding studies using peptides derived from rhoptry-associated protein 2 (RAP2)

Plasmodium falciparum rhoptry-associated proteins 1 (RAP1) and 2 (RAP2) are antigens presenting themselves as candidates for a subunit malaria vaccine. RAP2 protein, non-overlapping, consecutive peptides were synthesised and tested in red blood cell (RBC) binding assays to identify their receptor-ligand interaction in recognising RAP2 regions involved in the in vitro merozoite invasion process. Four high activity binding peptides (HABPs) were identified in the resulting 20 peptides. Peptides 26220 ((61)NHFSSADELIKYLEKTNINT(80)), 26225 ((161)IKKNPFLRVLNKASTTTHAT(180)) and 26229 ((241)RSVNNVISKNKTLGLRKRSS(260)) were located in the amino terminal and central part of the protein and HABP 26235 ((361)FLAEDFVELFDVTMDCYSRQ(380)) was located at the carboxy terminal. All these HABPs showed saturable binding and presented dissociation constants between 500 and 950 nM; the number of binding sites per RBC ranged from 48,000 to 160,000. High binding peptides' critical amino acids involved in RBC binding were determined by competition binding assays; their amino acids appear in bold in the sequences shown above. SDS-PAGE results showed that peptides 26220, 26225 and 26229 had at least two different sets of 62 and 42 kDa HABP receptors on RBCs and that peptide 26235 had at least two different sets of 77 and 62 kDa. HABPs inhibited in vitro merozoite invasion by between 54% and 94% at 200 microM, suggesting that these RAP2 peptides are involved in the in vitro P. falciparum invasion process.