Transferrin binding protein A (TbpA) is a TonB-dependent outer membrane protein expressed by pathogenic bacteria for iron acquisition from human transferrin. The N-terminal 160 residues (plug domain) of TbpA were overexpressed in both the periplasm and cytoplasm of Escherichia coli. We found this domain to be soluble and monodisperse in solution, exhibiting secondary structure elements found in plug domains of structurally characterized TonB-dependent transporters. Although the TbpA plug domain is apparently correctly folded, we were not able to observe an interaction with human transferrin by isothermal titration calorimetry or nitrocellulose binding assays. These experiments suggest that the plug domain may fold independently of the β-barrel, but extracellular loops of the β-barrel are required for ligand binding.
Bibliographical noteFunding Information:
We thank Travis Barnard, Reinhard Grisshammer, Alison Hickman, and Tara Kirby for critically reading the manuscript. M.O. was supported by the Engineering and Physical Sciences Research Council, UK, and by the NIDDK intramural program, NIH, USA; R.G. was supported by the NIDDK intramural program, NIH, USA; S.F. was supported by the Wellcome Trust, UK; A.R.G.’s work at the Health Protection Agency, Porton Down, was supported by the UK Department of Health; S.K.B. was supported by the Biotechnology and Biological Sciences Research Council, UK, and by the NIDDK intramural program, NIH, USA.
- HRP, horseradish peroxidase
- Human transferrin
- Iron transport
- Neisseria meningitidis
- PBS, phosphate-buffered saline
- TbpA, transferrin binding protein A
- TbpB, transferrin binding protein B
- Transferrin binding protein A
- hTf, human transferrin