Fig. 5. Alternating access model of choline import.

(A) Superposition of outward-open and occluded structures of LicB. Inverted repeats, TM1 to TM5 and TM6 to TM10, are indicated in blue and gray, respectively. Helices that undergo large conformational changes are shown in orange in the occluded state. (B) Schematic of the proposed transport cycle depicting the movements of TM3 and TM6 that close the extracellular entry pathway, whereas the predicted movements of TM1 and TM8 open the cytoplasmic exit pathway. Choline (orange) and protons enter the binding cavity in the outward-open state. LicB changes conformation to the occluded state before opening up to the cytoplasm for choline and proton release.