MOLECULAR VARIATION IN A NOVEL POLYMORPHIC ANTIGEN ASSOCIATED WITH PLASMODIUM-FALCIPARUM MEROZOITES

A cDNA clone encoding part of a novel polymorphic merozoite antigen fr om Plasmodium falciparum was isolated by screening a cDNA library with human immune serum from Papua New Guinea. Immunofluoresence microscop y and immunoblotting with affinity-purified antibodies recognized a hi ghly polymorphic antigen, Ag956, present in schizonts and merozoites. Biosynthetic labeling and immunoprecipitation experiments demonstrated that Ag956 is proteolytically cleaved during merozoite maturation. Th e complete genomic sequence of Ag956 from the D10 clone of P. falcipar um isolate FC27 encodes a secreted protein of calculated molecular mas s 43 243 that is very hydrophilic and contains a region of unusual hep tad repeats of the general structure AXXAXXX. This antigen has been na med the secreted polymorphic antigen associated with merozoites (SPAM) . The sequence of a second SPAM allele from the 3D7 clone of isolate N F54 reveals that the alanine heptad repeats and the hydrophilic C-term inal half of the protein are conserved. Variation among SPAM alleles i s the result of deletions and amino acid substitutions in non-repetiti ve sequences within and flanking the alanine heptad-repeat domain. Hep tad repeats in which the a and d position contain hydrophobic residues generate amphipathic cr-helices which give rise to helical bundles or coiled-coil structures in proteins. Thus, SPAM is the first example o f a P. falciparum antigen in which a repetitive sequence has features characteristic of a well-defined structural element.