ATP-driven conformational dynamics reveal hidden intermediates in a heterodimeric ABC transporter

Researchers have uncovered new insights into the inner workings of a type of transporter protein known as the heterodimeric type IV ABC transporter, specifically the TmrAB. This protein plays a crucial role in molecular machinery, similar to the human antigen transporter TAP. Traditionally, the dynamics of such proteins have been inferred from averaged measurements, but resolving individual molecule-level dynamics requires more advanced techniques.

Utilizing single-molecule Förster resonance energy transfer (smFRET), scientists were able to scrutinize the ATP-driven conformational dynamics of TmrAB. By attaching fluorophores to the nucleotide-binding domains and the periplasmic gate, they validated the reliability of these reporters in tracking conformational transitions. The results showed distinct ATP-free and ATP-bound states, with a clear quantification of how ATP shifts populations between these states at physiologically relevant concentrations.

The kinetic analysis of TmrAB revealed something quite surprising: the protein remains bound to ATP for a surprisingly long period, approximately 300 milliseconds. To delve deeper, researchers employed stabilization strategies to directly observe a previously hidden state where the protein's structure is outward-facing. This state was previously masked during the normal functioning of the transporter under turnover conditions.

These findings represent the first comprehensive single-molecule characterization of the TmrAB and set the stage for a quantitative framework to explore how ATP-coupled conformational dynamics operate in heterodimeric ABC transporters. This approach offers a more detailed understanding of these essential molecular machines, paving the way for future research into their biological functions and potential therapeutic targets.