Deletion and Substitution Analysis of theEscherichia coliTonB Q160 Region

Abstract

The active transport of iron siderophores and vitamin B 12 across the outer membrane (OM) of Escherichia coli requires OM transporters and the potential energy of the cytoplasmic membrane (CM) proton gradient and CM proteins TonB, ExbB, and ExbD. A region at the amino terminus of the transporter, called the TonB box, directly interacts with TonB Q160 region residues. R158 and R166 in the TonB Q160 region were proposed to play important roles in cocrystal structures of the TonB carboxy terminus with OM transporters BtuB and FhuA. In contrast to predictions based on the crystal structures, none of the single, double, or triple alanyl substitutions at arginyl residues significantly decreased TonB activity. Even the quadruple R154A R158A R166A R171A mutant TonB still retained 30% of wild-type activity. Up to five residues centered on TonB Q160 could be deleted without inactivating TonB or preventing its association with the OM. TonB mutant proteins with nested deletions of 7, 9, or 11 residues centered on TonB Q160 were inactive and appeared never to have associated with the OM. Because the 7-residuedeletion mutant protein (TonB⌬7, lacking residues S157 to Y163) could still form disulfide-linked dimers when combined with W213C or F202C in the TonB carboxy terminus, the TonB⌬7 deletion did not prevent necessary energy-dependent conformational changes that occur in the CM. Thus, it appeared that initial contact with the OM is made through TonB residues S157 to Y163. It is hypothesized that the TonB Q160 region may be part of a large disordered region required to span the periplasm and contact an OM transporter.The gram-negative cell envelope consists of an energized cytoplasmic membrane (CM) and a concentric, unenergized outer membrane (OM), separated by the aqueous periplasmic space. The OM is a diffusion barrier, protecting the cell from hydrophobic drugs, detergents, and degradative enzymes present in the environment while allowing the passive diffusion of nutrients smaller than 600 Da through the porins (56). The transport of Fe(III) siderophores or cobalamin across the OM requires the CM protonmotive force to be transduced into high-affinity transporters in the spatially separate OM. The TonB system in the CM (TonB, ExbB, and ExbD) plays a key role in energy transduction events at the OM (for recent reviews, see references 62, 76, and 77).TonB, ExbB, and ExbD form a complex in the CM through their transmembrane domains (5,22,28,31,33,34,48,60,67,73,74). TonB is present in the CM as a dimer (19, 68). The TonB/ExbB/ExbD ratio in the cell is 1:7:2, although it is not known if this value reflects the ratio of the three proteins in an energy transduction complex (23, 26). Unlike ExbB and ExbD, approximately one-third of the total cellular TonB is found associated with the OM following sucrose density gradient fractionation (50). Present data suggest that TonB achieves OM association through a shuttling process whereby the entire protein transits out of the CM following initial OM contact and associates entirely with t...

Identification of Functionally Important TonB-ExbD Periplasmic Domain Interactions In Vivo

“…The predicted unstructured nature of this region may facilitate conformational changes that permit the interaction of ExbD at multiple sites. The close interaction of ExbD with this unstructured region would support a previously proposed role for ExbD in the potential disorder-to-order transitions of this region of TonB (54).…”

Novel Colicin F Y of Yersinia frederiksenii Inhibits Pathogenic Yersinia Strains via YiuR-Mediated Reception, TonB Import, and Cell Membrane Pore Formation

“…The lowest sequence identity was found in the middle part of the TonB proteins of E. coli and Y. kristensenii, which was previously described as an important region for interaction with TonB boxes of colicins or receptors (7,20). Although deletion of Q160 from the TonB protein resulted in colicin-specific decreased susceptibility (78), deletion of seven amino acids (157S to Y163) from TonB caused complete resistance to all of the colicins tested (78). The authors hypothesized that the Q160 region may be a part of a larger region that is required for contact with the outer membrane receptor.…”

Taking the Escherichia coli TonB Transmembrane Domain “Offline”? Nonprotonatable Asn Substitutes Fully for TonB His20

“…Plasmids used in this study are listed in Table 1. TonB(H20X) substitutions and His relocation mutants were created on either a pKP325 (tonB) or pKP381 [tonB(H20A)] template as previously described (47).…”