TonB-dependent membrane receptors from bacteria have been analyzed in detergent-containing solution, an environment that may influence the role of ligand in inducing downstream interactions. We report reconstitution of FhuA into a membrane mimetic: nanodiscs. In contrast to previous results in detergent, we show that binding of TonB to FhuA in nanodiscs depends strongly on ferricrocin. The stoichiometry of interaction is 1:1 and the binding constant KD is ~200nM; an equilibrium affinity that is ten-fold lower than reported in detergent. FhuA in nanodiscs also forms a high-affinity binding site for colicin M (KD ~3.5nM), while ferricrocin renders FhuA refractory to colicin binding. Together, these results demonstrate the importance of the ligand in regulating receptor interactions and the advantages of nanodiscs to study β-barrel membrane proteins in a membrane-like environment.
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