The essential process of protein secretion is achieved by the ubiquitous Sec machinery. In prokaryotes, the drive for translocation through the SecYEG channel comes primarily from ATP hydrolysis by the cytosolic motor-protein SecA. However, the mechanism through which ATP hydrolysis is coupled to directional movement through SecYEG is unclear. Through a combination of all-atom MD simulations and single molecule FRET and biochemical assays, it is shown that ATP binding by SecA causes opening of the SecY-channel at long range, while substrates at the SecY-channel entrance feed back to regulate nucleotide exchange in SecA.
This two-way communication suggests a new 'Brownian ratchet' mechanism, whereby ATP binding and hydrolysis bias the direction of polypeptide diffusion. The model represents a solution to the problem of transporting inherently variable substrates such as polypeptides, and may underlie mechanisms of other motors that translocate proteins and nucleic acids.