HS-AFM single-molecule structural biology uncovers basis of transporter wanderlust kinetics.

The Pyrococcus horikoshii amino acid transporter Glt revealed, like other channels and transporters, activity mode switching, previously termed wanderlust kinetics. Unfortunately, to date, the basis of these activity fluctuations is not understood, probably due to a lack of experimental tools that directly access the structural features of transporters related to their instantaneous activity. Here, we take advantage of high-speed atomic force microscopy, unique in providing simultaneous structural and temporal resolution, to uncover the basis of kinetic mode switching in proteins. We developed membrane extension membrane protein reconstitution that allows the analysis of isolated molecules. Together with localization atomic force microscopy, principal component analysis and hidden Markov modeling, we could associate structural states to a functional timeline, allowing six structures to be solved from a single molecule, and an inward-facing state, IFS, to be determined as a kinetic dead-end in the conformational landscape. The approaches presented on Glt are generally applicable and open possibilities for time-resolved dynamic single-molecule structural biology.