When using these animations in any way, please acknowledge:
Matthew L. Walker, Stan A. Burgess, James R. Sellers, Fei Wang, John A. Hammer III, John Trinick & Peter J. Knight. Two-headed Binding of a Processive Myosin to F-actin. Nature, 405, 804-807 (2000).
Myosins are a family of motor proteins ubiquitous in animal cells. Most have two heads and a tail. The head comprises a motor domain that binds ATP and actin, and a so-called regulatory domain connecting the motor domain to the tail. The regulatory domain is thought to act as a lever arm, amplifying small changes in the motor into a large movement of the tail. Previous evidence had suggested that the heads move along actin filaments by a cyclical action of binding, changing shape and then detaching, using ATP as fuel. However, until now the initial attached state (the pre-power stroke position) had not been seen.
Myosin V is present in many animal cell types, and is particularly abundant in neurones. Defects in it have many consequences including neurological problems that can be fatal. It differs from myosin II which drives muscle contraction in having longer regulatory domains and a shorter tail. Each head also spends more time attached to actin during the cycle, which allows the molecule to take several steps along the actin without falling off. This is called processive motion.
Actin filaments are built of subunits arranged helically. Only if the myosin takes strides equal to the helical repeat can it move processively in a straight line. Otherwise it will take a helical path. We have analysed images in which both heads are attached to discover that the stride is typically of the correct length (36 nm) for linear motion.