Identification of Proteins from Plasmodium falciparum That Are Homologous to Reticulocyte Binding Proteins in Plasmodium vivax

Abstract

Plasmodium falciparum infections can be fatal, while P. vivax infections usually are not. A possible factor involved in the greater virulence of P. falciparum is that this parasite grows in red blood cells (RBCs) of all maturities whereas P. vivax is restricted to growth in reticulocytes, which represent only approximately 1% of total RBCs in the periphery. Two proteins, expressed at the apical end of the invasive merozoite stage from P. vivax, have been implicated in the targeting of reticulocytes for invasion by this parasite. A search of the P. falciparum genome databases has identified genes that are homologous to the P. vivax rbp-1 and -2 genes. Two of these genes are virtually identical over a large region of the 5 end but are highly divergent at the 3 end. They encode high-molecular-mass proteins of >300 kDa that are expressed in late schizonts and localized to the apical end of the merozoite. To test a potential role in merozoite invasion of RBCs, we analyzed the ability of these proteins to bind to mature RBCs and reticulocytes. No binding to mature RBCs or cell preparations enriched for reticulocytes was detected. We identified a parasite clone that lacks the gene for one of these proteins, showing that the gene is not required for normal in vitro growth. Antibodies to these proteins can inhibit merozoite invasion of RBCs.

Invasion of host cells by malaria parasites: a tale of two protein families

“…While PfRH1 as well as PfRH2a and 2b seem to locate to the rhoptries of the merozoite PfRH-4 seems to have a micronemal location (Kaneko et al, 2002), although recent evidence place it in the rhoptries (Stubbs et al, 2005). There is some evidence that at least a small proportion of PfRH2a is moved to the merozoite apical pole after schizont rupture (Triglia et al, 2001a), akin to the behaviour of Py235 (Oka et al, 1984;Sam-Yellowe, 1996). PFRH3 is considered to be a pseudogene and no corresponding protein is expressed during the erythrocytic cycle, although evidence suggests it may be expressed in sporozoites (Florens et al, 2002).…”

“…PfRH1 has been shown to directly bind a sialic acid containing trypsin-resistant receptor on the erythrocyte and antibodies raised against this protein inhibit merozoite invasion of trypsin-treated erythrocytes (Rayner et al, 2001). So far direct binding of PfRH2a or 2b to erythrocytes has not been demonstrated although there is evidence that antibodies can differentially inhibit invasion in some parasite strains, and certain peptide sequences from these proteins mediate stronger erythrocyte binding than others (see Table 1) (Triglia et al, 2001a;Ocampo et al, 2004). The most convincing evidence came from gene knockout experiments showing that PfRH2b recognizes a unique chymotrypsin-sensitive receptor and that it functions independently of other members (Duraisingh et al, 2003a).…”

“…These are the family of RBLs and the family of EBLs summarized in Table 1. The RBL superfamily includes P. yoelii 235 kDa rhoptry proteins (Py235) (Holder and Freeman, 1981), P. vivax reticulocyte-binding proteins (PvRBP-1 and 2) (Galinski et al, 1992) and the P. falciparum rhoptry protein homologues (PfRH1, PfRH2a, PfRH2b, PfRH3, PfRH4 and PfRH5; Tsuboi et al, 1994;Rayner et al, 2000;Taylor et al, 2001;Triglia et al, 2001a;Kaneko et al, 2002;Stubbs et al, 2005;Cowman and Crabb, 2006). EBL homologues include the DBP of P. vivax and P. knowlesi, the P. falciparum EBA175, BAEBL (EBA140), EBL1, JESEBL (EBA181) and PEBL (EBA165) (Camus and Hadley, 1985a;Peterson and Wellems, 2000;Mayer et al, 2001;Thompson et al, 2001;Triglia et al, 2001b;Lobo et al, 2003;Maier et al, 2003).…”

“…For P. falciparum, it had been difficult to establish whether members of RBL were also present because of low sequence homology and the lack of any cross-reactive immunological reagents with those known in other Plasmodium species. Once the P. falciparum genome sequence became available, six members of the RBL family were identified, the rhoptry protein homologue 1 (PfRH1) also named normocyte binding protein 1 (NBP1) (Rayner et al, 2001), the reticulocyte-binding protein homologues 2a and 2b (PfRBP-2a and 2b) (Rayner et al, 2000;Triglia et al, 2001a), the rhoptry protein homologue 3 (PfRH3) a possible pseudogene in a number of laboratory cultured parasite isolates , the reticulocytebinding homologue 4 (PfRH4) (Kaneko et al, 2002) and the reticulocyte-binding protein homologue 5 (PfRH5) (Cowman and Crabb, 2006). For this review and for the sake of uniformity we will name them PfRH1, PfRH2a, PfRH2b, PfRH3, PfRH4 and PfRH5 respectively.…”

Normocyte-binding protein required for human erythrocyte invasion by the zoonotic malaria parasite Plasmodium knowlesi

“…Plasmodium falciparum normocyte-binding proteins (NBPs) (13,14) are members of this family that includes four conventional RBL/RH proteins and one unconventional short RBL/RH protein (RH5) in this species (reviewed in ref. 5).…”