Neisseria meningitidis, a causative agent of
bacterial meningitis and
septicemia, obtains
transferrin-bound
iron by expressing two outer membrane-located
transferrin-binding proteins, TbpA and TbpB. A novel system was developed to investigate the interaction between
Tbps and human
transferrin. Copurified TbpA-TbpB, recombined TbpA-TbpB, and individual TbpA and TbpB were reconstituted into
liposomes and fused onto an HPA chip (BIAcore). All preparations formed stable monolayers, which, with the exception of TbpB, could be regenerated by removing bound
transferrin. The
ligand binding properties of these monolayers were characterized with surface plasmon resonance and shown to be specific for human
transferrin. Kinetic data for diferric human
transferrin binding showed that recombined TbpA-TbpB had K(a) and K(d) values similar to those of copurified TbpA-TbpB. Individual TbpA and TbpB also displayed K(a) values similar to those of copurified TbpA-TbpB, but their K(d) values were one order of magnitude higher. Chemical cross-linking studies revealed that TbpA and TbpB, in the absence of human
transferrin, formed large complexes with TbpA as the predominant species. Upon human
transferrin binding, a complex was formed with a molecular mass corresponding to that of a TbpB-human
transferrin heterodimer as well as a higher-molecular-mass complex of this heterodimer cross-linked to TbpA. This indicates that TbpA and TbpB form a functional meningococcal receptor complex in which there is cooperativity in the human
transferrin binding kinetics. However,
iron loss from the diferric human
transferrin-TbpA-TbpB complex was not greater than that from human
transferrin alone, suggesting that additional meningococcal transport components are involved in the process of
iron removal.