To further illustrate the properties of motor proteins, we consider myosin II, which moves along actin filaments in muscle cells during contraction. Other types of myosin can transport vesicles along actin filaments in the cytoskeleton. Myosin II and other members of the myosin superfamily are composed of one or two heavy chains and several light chains. The heavy chains are organized into three structurally and functionally different types of domains (Figure 3-24a).
The two globular head domains are specialized ATPases that couple the hydrolysis of ATP with motion. A critical feature of the myosin ATPase activity is that it is actin activated. In the absence of actin, solutions of myosin slowly convert ATP into ADP and phosphate. However, when myosin is complexed with actin, the rate of myosin ATPase activity is four to five times as fast as it is in the absence of actin. The actin-activation step ensures that the myosin ATPase operates at its maximal rate only when the myosin head domain is bound to actin. Adjacent to the head domain lies the a-helical neck region, which is associated with the light chains. These light chains are crucial for converting small conformational changes in the head into large movements of the molecule and for regulating the activity of the head domain. The rodlike tail domain contains the binding sites that determine the specific activities of a particular myosin.
The results of studies of myosin fragments produced by proteolysis helped elucidate the functions of the domains. X-ray crystallographic analysis of the S1 fragment of myosin II, which consists of the head and neck domains, revealed its shape, the positions of the light chains, and the locations of the ATP-binding and actin-binding sites. The elongated myosin head is attached at one end to the a-helical neck (Figure 3-24b). Two light-chain molecules lie at the base of the head domain shows that it has a curved, elongated shape and is bisected by a large cleft. The nucleotide-binding pocket lies on one side of this cleft, and the actin-binding site lies on the other side near the tip of the head. Wrapped around the shaft of the a-helical neck are the two light chains. These chains stiffen the neck so that it can act as a lever arm for the head. Shown here is the ADP-bound conformation.
head, wrapped around the neck like C-clamps. In this position, the light chains stiffen the neck region and are therefore able to regulate the activity of the head domain.
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