Contents of volume 200 and index to volume 20

Abstract

Synthetic filaments have been formed from myosins isolated from five different contractile systems: vertebrate smooth and skeletal muscle, the slime mold Physarum polycephalum, Amoeba proteus, and bovine blood platelets. It was found that the assembly of all synthetic filaments was inconsistent with the commonly accepted model of the myosin filament which has been developed for skeletal muscle. The common features of all synthetic filaments investigated were: (1) for lengths greater than about 1 μm, a regularly organized arrangement of “cross-bridges” in the central part of the filament rather than a cross-bridge-free “bare zone,” and (2) except for skeletal muscle myosin, cross-bridge-free “bare edge” of about 0.2 μm in length at the filament tips. The bare edges normally occurred on the opposite sides of the filaments such that the shorter filaments exhibited a characteristic asymmetric appearance. For filaments less than about 0.7 μm in length the two bare edges overlapped to form one central bare zone which in one projection was rhomboid in shape. The observed structure of the filaments was consistent with a helical assembly of antiparallel myosin dimers arranged obliquely with respect to the filament axis, giving rise to a mixed polarity of myosin heads along the whole filament length. Observations of native filaments obtained from both vertebrate smooth muscle and Amoeba proteus indicate that myosin assembles in vivo in essentially the same manner. It is concluded that the myosin filaments of smooth muscle and nonmuscle cells are basically different in their structural organization from the thick filaments of skeletal muscle and that this difference reflects the need for a different type of shearing interaction required by the less specialized organization of myofilaments in these systems.