Plasmodium vivax merozoite surface protein-3 contains coiled-coil motifs in an alanine-rich central domain

Plasmodium merozoites are covered with a palisade layer of proteins that ar e arranged as organized bundles or appear as protruding spikes by electron microscopy. Here we present a third Plasmodium vivax merozoite surface prot ein, PvMSP-3, which is associated with but not anchored in the merozoite me mbrane. Serum from a P, vivax immune squirrel monkey was used to screen a l ambda gt11 P, vivax genomic DNA (gDNA) library. Plaque-selected antibodies from clone no. 6.1, and rabbit antisera against its encoded protein, produc ed a pattern in immunofluorescence assays (IFAs) that is consistent with a localization at the surface of mature schizonts and free merozoites. Specif ic antisera also agglutinated merozoites and recognized a protein of 150 00 0 Da by SDS-PAGE. The complete msp-3 gene and flanking sequences were clone d from a P. vivax lambda Dash II gDNA library and also partly characterized by RACE (rapid amplification of cDNA ends). The immediate upstream sequenc e contains non-coding repeats and a putative protein-encoding open reading frame (ORF), which are also present on the msp-3 5'RACE gene product. Pvmsp -3 encodes a protein with a calculated mass of 89 573 Da, which has a poten tial signal peptide and a major central alanine-rich domain (31%) that exhi bits largely alpha-helical secondary structure and is flanked by charged re gions. The protein does not have a putative transmembrane domain or a conse nsus sequence for a glycosylphosphatidylinositol (GPI) anchor modification. However, the alanine-rich domain has heptad repeats that are predicted to form coiled-coil tertiary structures, which mediate protein-protein interac tions. PvMSP-3 is structurally related to P. falciparum MSP-3 and the 140 0 00 Da MSP of P, knowlesi. Characterization of PvMSP-3, thus, also begins to define a new interspecies family of evolutionarily related Plasmodium mero zoite proteins. (C) 1999 Published by Elsevier Science B.V. All rights rese rved.