CHARACTERIZATION OF A NEW 60-KDA APICAL PROTEIN OF PLASMODIUM-FALCIPARUM MEROZOITE EXPRESSED IN LATE SCHIZOGONY

Immunological cross-reactivity studies between the Apicomplexa Babesia divergens and Plasmodium falciparum allowed us to identify a P falcip arum 60 kDa protein (Pf60) using an antiserum directed against a B div ergens 37 kDa culture-derived exoantigen. In immunofluorescence assays (IFA), Pf60 appears as a doublet of fluorescent spots associated to t he apical pole of merozoites. The doublet co-locates with two rhoptry components: the protein RAP-1 and the 140/130/110 (105) kDa rhoptry pr otein complex suggesting the rhoptry location of Pf60. The biosynthesi s of Pf60, established by labeling experiments with [S-35]methionine o n synchronized cultures, and by immunofluorescence detection, occurred during late schizogony. The physico-chemical properties of Pf60, the absence of identified precursor forms and the absence of co-precipitat ion with other proteins indicated a new class of rhoptry protein. Pf60 was detected in all the different geographic P falciparum strains so far tested, with a slight variability in molecular mass ranging from 5 8 to 60 kDa. During the invasion process of erythrocytes by merozoites , the IFA showed the presence of the Pf60 in the apex of free merozoit es, but not in invading merozoite, as well as in new ring-infected ery throcytes. Furthermore, immunoprecipitation assays indicated the prese nce of Pf60 in the culture medium, and its absence in new ring-infecte d erythrocytes. All together these results suggest a possible involvem ent of the Pf60 protein in the invasion process.