ENERGY-DEPENDENT CHANGES IN THE GONOCOCCAL TRANSFERRIN RECEPTOR

The pathogenic Neisseria spp. are capable of iron utilization from hos t iron-binding proteins including transferrin and lactoferrin. Transfe rrin iron utilization is an energy-dependent, receptor-mediated event in which two identified transferrin-binding proteins participate. One of these proteins, TbpA, is homologous to the TonB-dependent family of outer membrane receptors that are required for high-affinity uptake o f vitamin B-12 and ferric siderophores. The 'TonB box' is a conserved domain near the amino-terminus of these proteins that has been implica ted in interaction with Tone. Interaction between a periplasmic domain of TonB and the TonB box allows energy transduction to occur from the cytoplasmic membrane to the energy-dependent receptor in the outer me mbrane. We created a Tone box mutant of gonococcal TbpA and demonstrat ed that its binding and protease accessibility characteristics were in distinguishable from those of gonococcal Ton system mutants. The prote ase exposure of the second transferrin-binding protein, TbpB, was affe cted by the energization of TbpA, consistent with an interaction betwe en these proteins. TbpB expressed by the de-energized mutants was read ily accessible to protease, similar to TbpB expressed in the absence o f TbpA. The de-energized mutants exhibited a marked decrease in transf errin diffusion rate, suggesting that receptor energization was necess ary for ligand release. We propose a model to explain the observed Ton -dependent changes in the binding parameters and exposures of TbpA and TbpB.