Paul R. Selvin, Elise Burmeister Getz and *Roger Cooke
Lawrence Berkeley National Laboratory, Berkeley CA 94720 and *University of California San Francisco, San Francisco, CA 94143.
We have measured distances between the neck and catalytic regions of myosin using luminescence resonance energy transfer. The technique extends the distance-range and accuracy of conventional fluorescence resonance energy transfer measurements. In the absence of actin, the distances are in agreement with the myosin crystal structure. In the presence of actin, the measurements indicate that myosin adopts a slightly more open configuration than without actin. Specifically, we find the distance between cys109 on gizzard light chain and cys707 in the catalytic domain to be 72 Å in the rigor complex in the absence of actin. This distance increases to 76 Å upon myosin binding to actin. We have also measured this distance as a function of nucleotide (ATP, ADP) and nucleotide analogs. The measurements support the notion that cys707 on the heavy chain is near, but not at, the fulcrum for myosin flexure. The measurements also indicate that a neck-swing occurs during the muscle powerstroke: fitting of the distances into the myosin crystal structure suggests a 24° neck-rotation and a 28 Å powerstroke, although the uncertainty in these numbers is large because cys707 is near the fulcrum.