Visualizing multistep elevator-like transitions of a nucleoside transporter

Membrane transporters move substrates across the membrane by alternating access of their binding sites to one side of the membrane at a time. An emerging model is the elevator mechanism in which a substrate-binding transport domain moves a large distance across the membrane. This mechanism has been characterized by a transition between two states but the conformational path leading to the transition is not yet known, largely because the available structural information has been limited to the two end states. Here we present crystal structures of a concentrative nucleoside transporter from Neisseria wadsworthii representing inward-facing, intermediate, and outward-facing states. Interestingly, we determined the structures of multiple intermediate conformations in which the transport domain is captured halfway through its elevator motion. Our structures present a trajectory of the conformational transition in the elevator model, revealing multiple intermediate steps and state-dependent conformational changes within the transport domain associated with the elevator-type motion.