The TonB_C domain is the well-characterised C-terminal region of the TonB receptor molecule. This protein is bound to an inner membrane-bound protein ExbB via a globular domain and has a flexible middle region that is likely to help in positioning the C-terminal domain into the iron-transporter barrel in the outer membrane [1]. TonB_C interacts with the N-terminal TonB box of the outer membrane transporter that binds the Fe3+-siderophore complex. The barrel of the transporter, consisting of 22 beta-sheets and an inside plug, binds the iron complex in the barrel entrance [2].

1: Outer membrane active transport: structure of the BtuB:TonB complex. Shultis DD, Purdy MD, Banchs CN, Wiener MC; Science. 2006;312:1396-1399. PMID:16741124

2: Recent insights into iron import by bacteria. Braun V, Hantke K; Curr Opin Chem Biol. 2011;15:328-334. PMID:21277822