ANTIBODIES TO A CONSERVED-MOTIF PEPTIDE SEQUENCE OF THE PLASMODIUM-FALCIPARUM THROMBOSPONDIN-RELATED ANONYMOUS PROTEIN AND CIRCUMSPOROZOITEPROTEIN RECOGNIZE A 78-KILODALTON PROTEIN IN THE ASEXUAL BLOOD STAGESOF THE PARASITE AND INHIBIT MEROZOITE INVASION IN-VITRO

Athrombospondin-related anonymous protein (TRAP) of the human malaria parasite Plasmodium falciparum shares highly conserved amino acid sequ ence motifs with the circumsporozoite protein of all plasmodia sequenc ed so far, as well as with unrelated proteins like thrombospondin and properdin. Although it was first described as an asexual blood stages protein, there has been some controversy about its expression in these stages. Pursuant to our interest in the conserved sequences within th e malaria antigens, we synthesized an 18-residue peptide (18-mer) repr esenting a conserved motif of TRAP and raised polyclonal antibodies ag ainst it. In an immunoblot assay in which we probed proteins from the asexual blood stages of the parasite, we found that this antibody reco gnized predominantly a 78-kDa protein in the whole parasite lysate. Fu rthermore, in another immunoblot, the recombinant TRAP constructs cont aining the conserved-motif sequence were distinctly recognized by the antipeptide antibodies, whereas a construct lacking the motif sequence was not, suggesting that the antibodies specifically cross-reacted wi th a protein which might be a TRAP-like protein present in the asexual blood stages of the parasite. Also, in an immunofluorescence assay, t his antibody brightly stained the acetone-fixed trophozoites of the pa rasite. Most significantly, anti-18-mer immunoglobulin G, as well as a ntipeptide antibody against a smaller (nonamer) construct representing the most conserved motif within the 18-mer, inhibited the merozoite i nvasion of erythrocytes in a dose-dependent manner. These results prov ide evidence of the expression of TRAP or a TRAP-like protein in the a sexual blood stages of the parasite and of a possible role of the cons erved motifs in the parasite-host cell interaction during the process of invasion.