Muscle contraction: challenges for synchrotron radiation

The molecular mechanism of muscle contraction remains one of the major unresolved problems in biology. Muscle contraction occurs when the two sets of thin (actin) and thick (myosin) filaments slide past each other, powered by adenosine triphosphate (ATP). The central problem to be elucidated is to find out how force generation or sliding in muscle is associated with major conformational changes in the two proteins, actin and myosin, when they interact with each other. X-ray diffraction is the only technique for pursuing the structural events in the contraction process with spatial resolution (1-60 nm) and time resolution (milliseconds or less) in a living cell. Because of its inherent weak diffractor of X-rays, the use of synchrotron radiation as an intense source was inevitable for diffraction studies of muscle. Recent synchrotron X-rays have extended the diffraction pattern to atomic resolution, providing an important structural basis for the molecular mechanism underlying the whole process of muscle contraction. The analysis of the diffraction pattern has been much more sophisticated in the benefit of the determination of the atomic structures of actin and myosin. For understanding the dynamic function of proteins and their interaction in a cell at an atomic level as an ultimate goal in structural biology, muscle research has been its best example. It will continue to be a challenging field in life science. In this article the authors' own research activities in the muscle field which have been performed using synchrotron radiation are described and future challenges are discussed.