Rapid evolution of an erythrocyte invasion gene family: the Plasmodium reichenowi Reticulocyte Binding Like (RBL) genes

Malarial merozoites use an array of ligands, including members of the Reticulocyte Binding Like (RBL) super-family of invasion proteins, to identify and invade erythrocytes. RBL family members are large Type I membrane anchored proteins expressed at the invasive end of merozoites that share homology with the Reticulocyte Binding Proteins 1 and 2 (PvRBP1 and 2) of Plasmodium vivax. Plasmodium species vary widely both in the number and sequence of their RBL genes, with the recently completed Plasmodium falciparum genome containing five RBL genes. Of these, three encode proteins shown to be involved in erythrocyte invasion, a fourth is a pseudogene, and the role of the fifth is as yet unclear. In order to identify sequence similarities and differences that may have functional implications for erythrocyte invasion as well as to gain insights into the recent evolutionary history of the P. falciparum RBL genes, we have sequenced all five corresponding RBL genes from the chimpanzee parasite Plasmodium reichenowi, which is the closest phylogenetic relative of P. falciparum, yet is unable to invade human erythrocytes. Two of the five P. falciparum RBL genes have highly conserved complete open reading frames in both species, while the other three genes show evidence of gene conversion and rapid evolution. The RBL super-family, therefore, appears to be surprisingly dynamic and divergent, implying that it is involved in species-specific aspects of erythrocyte recognition and invasion.