Why choose myofibrils to study muscle myosin ATPase?

Our objective is to propose an overview of the usefulness of skeletal myofibril as an experimental system for studying mechanochemical coupling of skeletal muscles and myosin ATPase activity. The myofibril is a true functional mini-muscle that is able to contract in the presence of ATP. It also contains the machinery necessary for the calcium sensitivity of the contraction. In the absence of calcium, myofibrillar ATPase activity is basal, no shortening occurs and no active force is developed. In the presence of calcium, myofibrillar ATPase is activated and myofibrils either shorten with no external load (native myofibrils) or contract isometrically (cross-linked myofibrils). With this organised system, both chemical and mechanical studies can be carried out. For a decade, our laboratory has been using rabbit psoas myofibrils for exploring myosin ATPase activity. The first challenge was to successfully apply rapid kinetic approaches, such as rapid-flow-quench, to this organised system. Another challenge was to work with myofibrils in cryoenzymic conditions, i.e. in the presence of organic solvents and at sub-zero temperatures. In this overview, we highlight differences between the myosin ATPase in organised systems (myofibrils or fibres) and that of contractile proteins in solution (S1 or actoS1) that we observed using these approaches. We discuss the importance of these differences in terms of mechanochemical coupling. It is concluded that great care should be taken when extrapolating mechanochemical properties of the contractile proteins in solution to the whole muscle.