FIGURE 17-25 Three common classes of myosins. Myosin I molecules consist of a head domain and a neck domain; a variable number of light chains is associated with the neck domain. Members of the myosin I class are the only myosins to have a single head domain. Some of these myosins are believed to associate directly with membranes through lipid interactions. Myosin II molecules have two head domains and two light chains per neck and are the only class that can assemble into bipolar filaments. Myosin V molecules have two head domains and six light chains per neck. They bind specific receptors (brown box) on organelles, which they transport. All myosins in these three classes move toward the (+) end of actin filaments.