FRET between donor-labeled EDANS-miClpP and acceptor-labeled Fl-LYssrA
initiates earlier during a ClpA-mediated translocation reaction for a
substrate probe that is adjacent to the COOH-terminal ssrA tag. ClpA (3
μM hexamer), miClpP (1.5 μM tetradecamer), either Fl-LYssrA (1.5
μM), and ATPγS (1 mM) were incubated together in reaction buffer at
25°C for 45 min, then rapidly mixed with an excess of ATP (10 mM) in
the stopped-flow apparatus. Each trace is the sum of 10 runs. The
difference in the final FRET efficiency reached with the two molecules
(18% vs. 24.5%, respectively) likely results from asymmetric binding
of the tagged substrate within the ClpP chamber and, hence, different
distances and relative orientations between the probes.