ELASTIC BENDING AND ACTIVE TILTING OF MYOSIN HEADS DURING MUSCLE-CONTRACTION

Muscle contraction is driven by a change in shape of the myosin head r eg;ion that links the actin and myosin filaments(1,2). Tilting of the light-chain domain of the head with respect to its actin-bound catalyt ic domain is thought to be coupled to the ATPase cycle(3-6). Here, usi ng X-ray diffraction and mechanical data from isolated muscle fibres, we characterize an elastic bending of the heads that is independent of the presence of ATP. Together, the tilting and bending motions can ex plain force generation in isometric muscle, when filament sliding is p revented. The elastic strain in the head is 2.0-2.7 nm under these con ditions, contributing 40-50% of the compliance of the muscle sarcomere . We present an atomic model for changes in head conformation that acc urately reproduces the changes in the X-ray diffraction pattern seen w hen rapid length changes are applied to muscle fibres both in active c ontraction and in the absence of ATP. The model predictions are relati vely independent of which parts of the head are assumed to bend or til t, but depend critically on the measured values of filament sliding an d elastic strain.