A LARGE AND DISTINCT ROTATION OF THE MYOSIN LIGHT-CHAIN DOMAIN OCCURSUPON MUSCLE-CONTRACTION

For more than 30 years, the fundamental goal in molecular motility has been to resolve force-generating motor protein structural changes. Al though low-resolution structural studies have provided evidence for fo rce generating myosin rotations upon muscle activation, these studies did not resolve structural states of myosin in contracting muscle. Usi ng electron paramagnetic resonance, we observed two distinct orientati ons of a spin label attached specifically to a single site on the ligh t chain domain of myosin in relaxed scallop muscle fibers. The two pro be orientations, separated by a 36 degrees + 5 degrees axial rotation, did not change upon muscle activation, but the distribution between t hem changed substantially, indicating that a fraction (17% +/- 2%) of myosin heads undergoes a large (at least 300) axial rotation of the my osin light chain domain upon force generation and muscle contraction. The resulting model helps explain why this observation has remained so elusive and provides insight into the mechanisms by which motor prote in structural transitions drive molecular motility.