Analysis of polypeptide movement in the SecY channel during SecA-mediated protein translocation

In bacteria most secretory proteins are transported across the plasma membrane by the interplay of the ATPase SecA with the translocation channel formed by the SecY complex; SecA uses cycles of ATP hydrolysis to "push" consecutive segments of a polypeptide substrate through the channel. Here we have addressed the mechanism of this process by following the fate of stalled translocation intermediates. These were generated by using a polypeptide substrate containing a bulky disulfide-bonded loop, thus preventing the final residues from passing through the channel. Protease protection experiments showed that the intermediates were stable in the presence of ATP and could complete translocation once the block was removed. The translocation intermediate was also stable when SecA associated with ATP gamma S, a poorly hydrolyzable ATP analog, or ADP plus AlF4, which mimics the transition state during ATP hydrolysis. In contrast, when SecA was in its ADP-bound state, the translocating polypeptide moved back into the cytosol, as indicated by the disappearance of the protected fragment. Backsliding was not significantly altered by deletion of the plug domain, a short helix in the center of the SecY channel, but it was slowed down when changes were introduced into the pore ring, the constriction of the hourglass-shaped channel. In all cases, backsliding was significantly slower than forward translocation. Together, these data suggest that SecA binds the polypeptide chain in its ATP state and releases it in the ADP state. The channel itself does not bind the polypeptide chain but provides "friction" that minimizes backsliding when ADP-bound SecA resets to "grab" the next segment of the substrate.