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The Central Role of Duffy Binding–Like (DBL) Domains in Interaction of Malaria Parasites with Host Receptors for Invasion and Cytoadherence
Summary: Dr. Chitnis studies receptor-ligand interactions involved in erythrocyte invasion by malaria parasites.
Erythrocyte invasion by Plasmodium sp. merozoites and cytoadherence of P. falciparum–infected erythrocytes to host capillaries are two important pathogenic mechanisms in malaria. A family of erythrocyte-binding proteins (EBPs), which includes P. vivax and P. knowlesi Duffy-binding proteins (PvDBP and PkDBP, respectively), mediates interaction with erythrocyte receptors during invasion. Variant antigens (also known as PfEMP-1) expressed on the surface of P. falciparum–infected erythrocytes bind to host endothelial receptors to mediate cytoadherence.
We have used transfection technology to disrupt the gene encoding PkDBP to study the protein's role in invasion. We demonstrate that the interaction of PkDBP with Duffy antigen mediates junction formation, a critical step during erythrocyte invasion. The receptor-binding domains of EBPs and PfEMP-1 map to conserved cysteine-rich domains that are referred to as Duffy binding–like (DBL) domains, after the first receptor-binding domains identified from PvDBP and PkDBP. DBL domains contain about 350 amino acids with 12–16 conserved cysteines. Using chimeric constructs and mild proteolysis in conjunction with functional binding assays, we provide evidence for a multidomain architecture for DBL domains from EBPs and PfEMP-1. The N-terminal region containing cysteines 1–4 of PvDBP and PkDBP forms a distinct subdomain that is nonfunctional. The region containing cysteines 5–12 of PvDBP and PkDBP forms another subdomain that is capable of receptor binding. Expression of various deletion constructs on the surface of COS cells followed by functional binding assays demonstrates that receptor-binding sites of DBL domains derived from EBPs and PfEMP-1 usually lie in the central region spanning the equivalent of cysteines 5–8 of PvDBP and PkDBP. Site-directed mutagenesis is being used to identify the receptor-binding residues within this region. Understanding the structure-function relation of the interaction of DBL domains with host receptors is key to the development of receptor-blocking strategies that inhibit invasion or reverse cytoadherence.
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