Identification of heparin as a ligand for the A-domain of Plasmodium falciparum thrombospondin-related adhesion protein

Thrombospondin-related adhesion protein (TRAP) is a Plasmodium falciparum transmembrane protein that is expressed within the micronemes of sporozoites, and is implicated in host cell invasion and motility. Contained within the extracellular region of TRAP is an A-domain, a module found in a number of membrane, plasma and matrix proteins, that is often involved in ligand recognition. In order to determine the role of the TRAP A-domain, it has been expressed as a glutathione S-transferase fusion protein and its ligand binding compared with that of other characterised glutathione S-transferase A-domain fusion proteins. Using a solid phase assay to screen for binding to known A-domain ligands, the TRAP A-domain was found to bind heparin. Binding to heparin appeared to be specific as it was saturable, and was inhibited by soluble heparin, fucoidan and dextran sulfate, but not by other negatively charged sulfated glycosaminoglycans such as chondroitin sulfates. Furthermore, unlike some A-domain ligand interactions, the A-domain of both TRAP and the leukocyte integrin, Mac-1, bound to heparin in the absence of divalent cations. It has been shown previously that another domain within TRAP, which is homologous to region II-plus of circumsporozoite protein, binds to sulfatide and to heparan sulfate on the immortalised hepatocyte line HepG2. The TRAP A-domain also bound to sulfatide and to HepG2 cells. Thus the A-domain shares certain binding properties already attributed to the region II-plus-like domain of TRAP, and may contribute to the binding of TRAP to heparan sulfate on hepatocytes.