Abstract
Background Plasmodium vivax merozoites specifically invade reticulocytes. Until recently, two reticulocyte-binding proteins (Pvrbp1 and Pvrbp2) expressed at the apical pole of the P. vivax merozoite were considered to be involved in reticulocyte recognition. The genome sequence recently obtained for the Salvador I (Sal-I) strain of P. vivax revealed additional genes in this family, and in particular Pvrbp2a, Pvrbp2b (Pvrbp2 has been renamed as Pvrbp2c) and two pseudogenes Pvrbp2d and Pvrbp3. It had been previously found that Pvrbp2c is substantially more polymorphic than Pvrbp1. The primary goal of this study was to ascertain the level of polymorphism of these new genes.Methodology/Principal FindingsThe sequence of the Pvrbp2a, Pvrbp2b, Pvrbp2d and Pvrbp3 genes were obtained by amplification/cloning using DNA purified from four isolates collected from patients that acquired the infection in the four cardinal regions of Thailand (west, north, south and east). An additional seven isolates from western Thailand were analyzed for gene copy number variation. There were significant polymorphisms exhibited by these genes (compared to the reference Sal-I strain) with the ratio of mutations leading to a non-synonymous or synonymous amino acid change close to 3∶1 for Pvrbp2a and Pvrbp2b. Although the degree of polymorphism exhibited by these two genes was higher than that of Pvrbp1, it did not reach the exceptional diversity noted for Pvrbp2c. It was interesting to note that variations in the copy number of Pvrbp2a and Pvrbp2b occurred in some isolates.Conclusions/SignificanceThe evolution of different members of the Pvrbp2 family and their relatively high degree of polymorphism suggests that the proteins encoded by these genes are important for parasite survival and are under immune selection. Our data also shows that there are highly conserved regions in rbp2a and rbp2b, which might provide suitable targets for future vaccine development against the blood stage of P. vivax.
Highlights
Plasmodium spp. merozoites invade the host red cell via a multistep invasion process [1]
Parasite species differ in the preference they exhibit with respect to the type of red blood cells they can invade, for example P. malariae is almost exclusively observed in normocytes, while P. vivax is confined to reticulocytes
Investigations aiming to define the functional domains of Pvrbp genes or their receptors on the reticulocyte were hampered by the fact that a practical invasion assay for P. vivax invasion have been developed only recently [15], and that the two genes are quite large in size
Summary
Plasmodium spp. merozoites invade the host red cell via a multistep invasion process [1]. For P. vivax two proteins expressed at the apical pole of the merozoite have been identified and implicated in reticulocyte recognition/selection, and were named reticulocyte binding proteins (RBP) [2]. Genes related to those coding for the RBPs of P. vivax (Pvrbp) were found in Plasmodium species that infect humans [3,4,5,6], simians [7,8,9] and rodents [10,11,12]. The primary goal of this study was to ascertain the level of polymorphism of these new genes
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