Plasmodium vivax, P. cynomolgi, and P. knowlesi: identification of homologue proteins associated with the surface of merozoites.

We have identified a Plasmodium vivax merozoite surface protein (MSP) that migrates on SDS-polyacrylamide gels at a Mr of about 185 kDa. This protein was recognized by a P. vivax monoclonal antibody (mAb) that localizes the protein by immunofluorescence to the surface of merozoites and also immunoprecipitates this protein from NP-40 detergent extracts of [35S]methionine metabolically radiolabeled P. vivax schizonts. The P. vivax MSP does not become biosynthetically radiolabeled with [3H]glucoamine, [3H]myristate, [3H]palmitate, or [3H]mannose, indicating that this P. vivax MSP is not posttranslationally modified and bound to the merozoite membrane by a glycosylphosphatidylinositol (GPI) lipid anchor. Thus, in this respect, this protein is different from members of the MSP-1 protein family and from MSP-2 and MSP-4 of P. falciparum. The mAb cross-reacts with and outlines the surface of P. cynomolgi merozoites and immunoprecipitates a 150-kDa P. cynomolgi homologue. The mAb was used as an affinity reagent to purify the native homologous MSP from NP-40 extracts of P. cynomolgi mature schizonts in order to develop a specific polyclonal antiserum. The resulting anti-PcyMSP rabbit antiserum cross-reacts strongly with the P. vivax 185-kDa MSP and also recognizes an analogous 110-kDa protein from P. knowlesi. We have determined via an immunodepletion experiment that the 110-kDa P. knowlesi MSP corresponds to the PK 110 protein partially characterized earlier (Perler et al. 1987). The potential of P. vivax MSP as a vaccine candidate was addressed by conducting in vitro inhibition of erythrocyte invasion assays, and the IgG fraction of both the P. vivax MSP mAb and the P. cynomolgi MSP rabbit antiserum significantly inhibited entry of P. vivax merozoites. We denote, on a preliminary basis, these antigenically related merozite surface proteins PvMSP-185, PcyMSP-150, and PkMSP-110.