(A) The controlled phosphorylation of one of the two light chains has at least two effects in vitro:it causes a change in the conformation of the myosin head, exposing its actin-binding site, and it releases the myosin tail from a "sticky patch" on the myosin head, thereby allowing the myosin molecules to assemble into short bipolar filaments. The enzyme responsible for this phosphorylation (myosin light-chain kinase) is described later in connection with smooth muscle (see Figure 16-98). (B) Negatively stained short filaments of myosin-II that have been induced to assemble by the phosphorylation of their light chains. (B, courtesy of John Kendrick-Jones.)