Conserved Residues Ser¹⁶ and His²⁰ and Their Relative Positioning Are Essential for TonB Activity, Cross-linking of TonB with ExbB, and the Ability of TonB to Respond to Proton Motive Force

The cytoplasmic membrane protein TonB couples the proton electrochemical potential of the cytoplasmic membrane to transport events at the outer membrane of Gram-negative bacteria. The amino-terminal signal anchor of TonB and its interaction with the cytoplasmic membrane protein ExbB are essential to this process. The TonB signal anchor is predicted to form an α-helix, with a conserved face comprised of residues Ser¹⁶, His²⁰, Leu²⁷, and Ser³¹. Deletion of either Ser¹⁶ or His²⁰ or of individual intervening but not flanking residues rendered TonB inactive and unable to assume a proton motive force-dependent conformation. In vivo formaldehyde cross-linking experiments revealed that the ability of this subset of mutants to form a characteristic heterodimer with ExbB was greatly diminished. Replacement of residues 17-19 by three consecutive alanines produced a wild type TonB allele, indicating that the intervening residues (Val, Cys, and Ile) contributed only to spacing. These data indicated that the spatial relationship of Ser¹⁶ to His²⁰ was essential to function and suggested that the motif HXXXS defines the minimal requirement for the coupling of TonB to the cytoplasmic membrane electrochemical gradient. Deletion of Trp¹¹ resulted in a TonB that remained active yet was unable to cross-link with ExbB. Because Trp ¹¹ was demonstrably not involved in the actual cross-linking, these results suggest that the TonB/ExbB interaction detected by cross-linking occurred at a step in the energy transduction cycle distinct from the coupling of TonB to the electrochemical gradient.