Point Mutations in TbpA Abrogate Human Transferrin Binding in Neisseria gonorrhoeae.

TonB-dependent transporters (TDTs) are essential proteins for metal acquisition, an important step in the growth and pathogenesis of many pathogens, including Neisseria gonorrhoeae, the causative agent of gonorrhea. There is currently no available vaccine for gonorrhea; TDTs are being investigated as vaccine candidates because they are highly conserved and expressed . Transferrin binding protein A (TbpA) is an essential virulence factor in the initiation of experimental infection in human males and functions by acquiring iron upon binding to host transferrin (human transferrin [hTf]). The loop 3 helix (L3H) is a helix finger that inserts into the hTf C-lobe and is required for hTf binding and subsequent iron acquisition. This study identified and characterized the first TbpA single-point substitutions resulting in significantly decreased hTf binding and iron acquisition, suggesting that the helix structure is more important than charge for hTf binding and utilization. The D355P Δ and A356P Δ mutants demonstrated significantly reduced hTf binding and impaired iron uptake from Fe-loaded hTf; however, only the A356P Δ mutant was able to grow when hTf was the sole source of iron. The expression of was able to restore function in all mutants. These results implicate both D355 and A356 in the key binding, extraction, and uptake functions of gonococcal TbpA.