High-speed atomic force microscopy reveals a three-state elevator mechanism in the citrate transporter CitS

Abstract

Significance As cellular membranes are impermeable to most molecules, transporter proteins are typically present in the membrane to transport small molecules in or out of the cell. Due to the small, nanometer size of these transporters, it is challenging to study their transport mechanism. Here, we use advanced microscopy approaches to study in real time and at the single-molecule level the mode of action of the dimeric CitS tranpsorter. Using high-speed atomic force microscopy, we visualize the dynamic, elevator-like movement of the transporter, and we reveal that the two protomers move independently. We also discovered an intermediate state, reminiscent of another, unrelated transporter, indicating that independent evolutionary pathways have led to similar three-state elevator mechanisms.