Contraction of reconstituted Dictyostelium cytoskeletons: an apparent role for higher order associations among myosin filaments.

A large number of cellular functions require assembly of actin and myosin and coordinated interactions between the resulting filaments. To better understand the structure and function of one such contractile assembly, we have begun fractionation and reconstitution studies of Dictyostelium cytoskeletons. Isolated cytoskeletons rapidly contracted when mixed with Mg-ATP, and myosin II was essential for this since myosin-depleted (stripped) cytoskeletons failed to contract. Dictyostelium, Acanthamoeba, or skeletal muscle myosins bound to stripped cytoskeletons with equal efficiency, and the Mg-ATPase of all three myosins was stimulated by the cytoskeleton-associated actin. Near neutral pH, however, only the homologous system reconstituted with Dictyostelium myosin contracted, despite the fact that under the same conditions all three myosins bound to myosin-depleted (ghost) muscle myofibrils and restored contractility. Individual Dictyostelium myosin thick filaments have a strong tendency to aggregate and associate end-to-end, and this may be important for functional contraction of cytoskeletons. This suggestion is supported by the observation that under conditions where individual Acanthamoeba myosin filaments aggregated, reconstituted cytoskeletons contracted. None of the solution conditions tested caused rabbit muscle myosin filaments to aggregate or to contract cytoskeletons. Thus higher order associations among individual myosin filaments may be essential for some types of cell motility.