Fig. 5. Proton binding stabilizes an outward-closed conformation.

a Spin label pairs (red spheres) are depicted on the periplasmic side of the IF crystal structure. A salt bridge between Asp60 and Arg289 keeps the transporter in a closed conformation. b Raw DEER decays and fits (left) are presented for the distance distributions P(r) in lipid nanodiscs (middle) and in LMNG micelles (right). The distributions predicted from the IF crystal structure are shaded gray.