Structural Dynamics of the Myosin Relay Helix Resolved by DEER and Time-Resolved FRET

We have used DEER (double electron-electron resonance) and TR-FRET (time-resolved fluorescence resonance energy transfer) to study conformational changes within the myosin II relay helix. Major structural changes during the myosin II ATPase cycle take place in the force-generating domain. Crystal structures show that the converter domain, the relay helix, and SH1 helix have different conformations in the proposed pre- and post- powerstroke structural states of myosin. This dramatic structural change is detected in solution studies by intrinsic fluorescence [1, 2] and EPR of a spin label attached to SH1 [3]. In the present study, we focus on the relay helix as a crucial structural element involved in coupling between the force-generating domain and the nucleotide-binding pocket. Cysteine mutations were introduced into a Cys-lite construct of Dictyostelium discoideum (Dicty) myosin in the lower 50k domain (either D515C or A639C) and the C-terminal end of the relay helix (K498C). These constructs were selectively modified with either MSL/MSL or IAEDANS/DABCYL pairs, and the distance between probes was measured in different myosin conformations trapped with nucleotides or nucleotide analogs Two conformations of the relay helix (with distinct probe-to-probe distances, presumably corresponding to the “straight” and “bent“ states of the relay helix) were resolved. Observed distances were in good agreement with existing crystal structures, but at least two distinct structural states were present in certain biochemical states (e.g, with bound ADP.BeF3, ADP.Vi, ADP.AlF4). The mole fraction of the “bent” conformation was higher with post-hydrolysis analogs (ADP.Vi, ADP.AlF4) bound at the active site. Our results reveal structural rearrangements within a single subdomain of myosin and provide insights into the coupling between ATP binding and changes in the force-generating region.