A single power stroke by ATP binding drives substrate translocation in a heterodimeric ABC transporter.

ATP-binding cassette (ABC) transporters constitute the largest family of primary active transporters, responsible for many physiological processes and human maladies. However, the mechanism how chemical energy of ATP facilitates translocation of chemically diverse compounds across membranes is poorly understood. Here, we advance the quantitative mechanistic understanding of the heterodimeric ABC transporter TmrAB, a functional homolog of the transporter associated with antigen processing (TAP) by single-turnover analyses at single-liposome resolution. We reveal that a single conformational switch by ATP binding drives unidirectional substrate translocation. After this power stroke, ATP hydrolysis and phosphate release launch the return to the resting state, which facilitates nucleotide exchange and a new round of substrate binding and translocation. In contrast to hitherto existing steady-state assays, our single-turnover approach uncovers the power stroke in substrate translocation and the tight chemomechanical coupling in these molecular machines.