DISCRIMINATION BETWEEN APO AND IRON-LOADED FORMS OF TRANSFERRIN BY TRANSFERRIN-BINDING PROTEIN-B AND ITS N-TERMINAL SUBFRAGMENT

Many pathogens of the Pasteurellaceae and Neisseriaceae possess a surf ace receptor that binds transferrin (Tf) as an initial step in an iron acquisition process. This receptor is comprised of two proteins, tran sferrin binding protein A (TbpA) and transferrin binding protein B (Tb pB). Since the ability to recognize the iron-loaded form of Tf prefere ntially would be a useful attribute of these receptors, we examined th is property in a number of bacterial species. In solid-phase binding a ssays with isolated membranes, only the receptor from Moraxella catarr halis was capable of preferentially binding iron-loaded Tf. In a compe titive affinity isolation assay which enabled us to resolve TbpA and T bpB, TbpA from all tested species was shown to bind both apo and iron- loaded Tf. Under these assay conditions TbpB from M. catarrhalis, Haem ophilus somnus and Pasteurella haemolytica discriminated between apo a nd hole Tf, whereas TbpB from Neisseria meningitidis showed no discrim ination. The ability of TbpB from N. meningitidis to bind iron-saturat ed hTf preferentially became evident in a TbpA(-) background or by usi ng recombinant TbpB. In binding assays with recombinant fusion protein s, both intact TbpB and the N-terminal half of TbpB from all the teste d species preferentially bound Fe-loaded Tf, indicating that this may be a conserved mechanism by which these organisms optimize their abili ty to acquire iron. (C) 1998 Academic Press.