Unfolding the Luminal Domain of BtuB in a Native Bilayer

Gram-negative bacteria are characterized by the presence of an asymmetric, lipopolysaccharide-containing outer-membrane (OM). While the OM allows free diffusion of small molecules across it through open porins, importing large molecules (> ∼600 Da) is challenging. TonB-dependent transporters, a class of OM proteins, have evolved to aid the import of large, scarce nutrients by coupling to an energy-driven complex in the inner membrane. These transporters possess a structurally conserved luminal domain that blocks the transmembrane barrel in its closed state. Now, by simulating the TonB-dependent transporter BtuB in its native OM, we have quantified the effects of the highly charged lipopolysaccharides on unfolding of the luminal domain. Shown to be required for opening, we compare the unfolding pathway to previous simulations in a symmetric bilayer. Finally, we relate unfolding to a structural shift of the luminal domain in the ligand-bound state that signals occupancy.