The crystal structure of P. knowlesi DBPα DBL domain and its implications for immune evasion

Abstract

Plasmodium vivax invasion of human erythrocytes requires that the ligand domain of the Duffybinding protein (DBP) recognize its cognate erythrocyte receptor, making DBP a potential target for therapy. The recently determined crystal structure of the orthologous DBP ligand domain of the closely related simian malaria parasite Plasmodium knowlesi provides insight into the molecular basis for receptor recognition and raises important questions about the mechanism of immune evasion employed by the malaria parasite. Plasmodium merozoite invasion is a multi-step processInvasion of erythrocytes by malaria parasites is a multi-step process involving discrete parasite ligands and host cell receptors [1]. Duffy binding protein (DBP), the first such ligand identified in micronemes of invasive malaria merozoites [2], is absolutely vital for the invasion process of Plasmodium vivax [3]. Cysteine-rich region II of the DBP comprises the prototypical Duffy binding like (DBL) ligand domain [4,5], which is also found in other erythrocyte binding proteins (EBA-175, BAEBL, JESEBL) and in cytoadherence proteins (PfEMP-1) [6]. Although the putative ligand domains of these paralogues have <30% sequence identity, these cysteine-rich regions share a core set of conserved residues (e.g., cysteines and aromatic amino acids) believed to be structurally and functionally important. DBL domains of both the human parasite P. vivax DBP and simian parasite P. knowlesi DBPα interact with the Duffy antigen receptor for chemokines (DARC) [7] on the erythrocyte surface, leading to formation of a tight junction necessary for invasion. The crystal structure of the P. knowlesi DBPα DBL domain recently reported by Singh et al provides exciting insights into the functional character of the P. vivax DBP [8]. Plasmodium knowlesi α DBL structureThe overall structure of the P. knowlesi DBPα DBL is similar to that of the F1 and F2 DBL domains of EBA-175 [9]. All twelve conserved cysteines of the P. knowlesi DBPα DBL domain are involved in intradomain disulfide bridges that delimit three DBL subdomains in the backbone, which forms a 'boomerang-shaped unit'. The pattern of disulfide bonding is identical between the P. knowlesi DBPα DBL and the F1 and F2 DBLs of P. falciparum EBA-175, although the F2 has an additional disulfide bridge. Subdomains 1, 2, and 3 have two, one and three disulfide bonds, respectively, and are comprised of twelve alpha helices (Fig. 1). Residues 15-52 form a random-coil stretch that makes up subdomain 1. The region between subdomains 1 and 2 (residues 53-63) is disordered and missing from the crystal structure, but is predicted to form a flexible linker. The 'β finger' motifs that facilitate Subdomain 2 (residues 64-180) and subdomain 3 (residues 186-307) each contain six alpha helices and are attached by a short linker segment. Subdomain 3 forms a large loop stabilized by three disulfide bridges with alpha helix 8 atop alpha helices 7 and 9; however, the functional role of the subdomain 3 structure is unclear. Proposed DARC Re...

Naturally Acquired Antibodies to Plasmodium vivax Duffy Binding Protein (DBP) in Rural Brazilian Amazon

“…6,43 In fact, individuals briefly exposed to P. vivax developed anti-DBP inhibitory antibodies that are biased toward a specific DBP II variant. 44 Although we cannot rule out the possibility that polymorphisms at DBP II could contributed to the relatively low frequency (@30%) of inhibitory antibodies among long-term residents in Acre, it seems unlikely because 1) to reduce the potential effects of DBP polymorphism on antibody recognition, we used two Individual results of enzyme-linked immunosorbent assay (ELISA) for IgG antibodies to Duffy binding protein (DBP) in consecutive serum samples collected from 48 malaria-exposed subjects from Acre with one or more laboratory-confirmed infections with Plasmodium vivax during a cohort study in Acre, Brazil.…”

“…6,7 The DBP plays a major role in red blood cell invasion by P. vivax ; blocking DBP binding to the Duffy antigen/receptor for chemokines (DARC) reduces the parasite's ability to invade new erythrocytes. [8][9][10] Binding domains of DBP is located in the N-terminal cysteine-rich region II (DBP II ), which contains 330 amino acids.…”

Naturally acquired inhibitory antibodies toPlasmodium vivaxDuffy binding protein are short-lived and allele-specific following a single malaria infection

“…Another line of evidence suggests that those few polymorphic residues surrounding the DARC binding domain might elude binding of inhibitory antibody [7,16]. The second model seems to explain why antibodies to DBP can inhibit reticulocyte invasion by P. vivax effectively [22].…”

“…Invasive merozoites are believed to sequester microneme proteins until merozoites contact the target erythrocyte, presumably as a mechanism to reduce exposure of DBP to immune inhibition [16]. Currently, available data on humoral immune responses to DBP in human populations demonstrate that anti-DBP antibodies increase with exposure to P. vivax [17][18][19][20], and this immune response includes antibody activity that blocks adherence of DBPII to its receptor on erythrocytes [18,21].…”

Genetic Variation among Plasmodium vivax Isolates Adapted to Non-Human Primates and the Implication for Vaccine Development

“…7,30 The recently determined crystal structure of the P. knowlesi DBPα DBL domain 31 has proven suitable for modeling three-dimensional structures of other DBL domains, such as P. vivax DBP. 32–34 All but one of the DBP polymorphic residues of the P. vivax primate isolates (Figure 1A) appear as surface-exposed residues, and although scattered over the entire three-dimensional structure, polymorphisms occur most frequently within the central portion of region II necessary for receptor recognition. Perhaps most importantly in terms of vaccine development are amino acid differences immediately adjacent to key residues implicated in receptor recognition (Figure 4) and different from Salvador I, which includes residues of Palo Alto (N417K, L424I).…”