Dynamic interplay of membrane‐proximal POTRA domain and conserved loop L 6 in O mp85 transporter FhaC

Summary Omp85 transporters mediate protein insertion into, or translocation across, membranes. They have a conserved architecture, with POTRA domains that interact with substrate proteins, a 16‐stranded transmembrane β barrel, and an extracellular loop, L 6, folded back in the barrel pore. Here using electrophysiology, in vivo biochemical approaches and electron paramagnetic resonance, we show that the L 6 loop of the O mp85 transporter FhaC changes conformation and modulates channel opening. Those conformational changes involve breaking the conserved interaction between the tip of L 6 and the inner β‐barrel wall. The membrane‐proximal POTRA domain also exchanges between several conformations, and the binding of FHA displaces this equilibrium. We further demonstrate a dynamic, physical communication between the POTRA domains and L 6, which must take place via the β barrel. Our findings thus link all three essential components of O mp85 transporters and indicate that they operate in a concerted fashion in the transport cycle.