Recognition of a Mr 56K Glycoprotein on the Surface of Plasmodium Falciparum Merozoites by Mouse Monoclonal Antibodies

Hybridomas were prepared from mice repeatedly injected with disrupted Plasmodium falciparum (FVO isolate) schizonts and merozoites. Antibodies secreted by two of these hybridomas were shown by immunoelectron microscopy to bind to the surface of merozoites from the FVO isolate. These monoclonal antibodies (McAb) reacted with the FVO and Geneva isolates by an indirect fluorescence antibody test (IFAT) and immunoprecipitated a protein of relative molecular weight (Mr) 56K from both isolates. The 56K protein could be labeled with [35S] methionine and [3H]glucosamine. Glycosidase treatment of the affinity-purified polypeptide proved that the [3H]glucosamine had been incorporated into sugar side chains and that this protein (called gp56) was glycosylated. The anti-gp56 McAb did not react by IFAT or immunoprecipitation with four isolates (Honduras I, Indochina I, Tanzania I, and Kenya) that lack gp56 but contain major glycoproteins of Mr 50K. Antibodies from an Aotus monkey immune to the FVO isolate immunoprecipitated gp56 from both the FVO and Geneva isolates, but did not immunoprecipitate the 50K glycoproteins from the other four isolates. Extraction experiments conducted with the nonionic detergent Triton X-114 indicate that some of the gp56 molecules are hydrophilic and that the others are either hydrophobic or interact with hydrophobic molecules. These results, together with the electron microscopic data, suggest that the hydrophilic gp56 is a component of the extracellular matrix and that the hydrophobic gp56 may be associated with the plasma membrane of the merozoite.