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 t ransmembrane protein that is expressed within the micronemes of sporozoites , and is implicated in host cell invasion and motility. Contained within th e extracellular region of TRAP is an A-domain, a module found in a number o f membrane, plasma and matrix proteins, that is often involved in ligand re cognition. 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 bin ding 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. Bindin g to heparin appeared to be specific as it was saturable, and was inhibited by soluble heparin, fucoidan and dextran sulfate, but not by other negativ ely charged sulfated glycosaminoglycans such as chondroitin sulfates. Furth ermore, 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 diva lent cations. It has been shown previously that another domain within TRAP, which is homologous to region II-plus of circumsporozoite protein, binds t o sulfatide and to heparan sulfate on the immortalised hepatocyte line HepG 2. 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 h eparan sulfate on hepatocytes. (C) 1999 Elsevier Science B.V. All rights re served.