The role of myosin I and II in cell motility.

It has been recognized since the turn of the century that cell motility by non-muscle cells requires virtually continuous restructuring of the cytoskeleton (see refs [1-4]). It is also clear that cell motility requires a mechanism for converting chemical energy into mechanical work. The proteins actin and myosin, two important constituents of the cytoskeleton, have been postulated to act as the chemicomechanical transducer in motile cells. Central to their role as a force generating mechanism in motile cells is the ability of myosin (a) to hydrolyze ATP when it interacts with actin and (b) to form filaments. Recent studies on mammalian cells and on the cellular slime mold Dictyostelium discoideum have shed light and at the same time raised questions regarding the involvement of myosin in cell motility. Moreover, they have demonstrated the presence of two types of myosins, called myosin II and myosin I, that have unique biochemical and regulatory properties and that may play different roles in mediating cell motility. In this chapter we will discuss the properties of these two myosins and then describe what is known about their involvement in Dictyostelium and mammalian cell motility.