Paramyosin and the filaments of molluscan “catch” muscles

A method has been developed for the partial separation of the filaments of molluscan muscles. A fraction has been obtained which consisted mainly of actin filaments many of which were attached to dense bodies. When complexed with rabbit heavy meromyosin, the actin filaments showed arrowhead structures which appeared to have opposite polarity at either end of the dense bodies. The other fraction consisted of very long and thick filaments containing essentially all of the paramyosin and the myosin in the muscle. Some actin filaments were also present in this fraction. When myosin solubilization was prevented, myosin filaments similar to those of vertebrate striated muscle were rarely found in either of the two fractions. Myosin could be selectively extracted from the thick filaments without the solubilization of paramyosin; extraction of paramyosin, however, was always accompanied by the solubilization of myosin. The removal of myosin changed the surface appearance of the thick filaments: a characteristic pattern of darkly staining nodes or gap regions in the paramyosin filament became visible in negatively stained preparations. The nodes have a roughly triangular shape defining the polarity of the structure. A number of filaments were found which showed a reversal of polarity along their length. These results lead to the conclusion that paramyosin forms a bipolar core of the thick filament which is covered by a surface layer of myosin. The specific interactions between paramyosin and myosin could be demonstrated structurally and enzymically. Molluscan myosin formed filaments resembling those of rabbit myosin when precipitated in vitro. Its assembly in the molluscan thick filament is therefore determined by the underlying paramyosin core. Paramyosin greatly inhibited the actin activated ATPase of myosin in a selective manner when these proteins were mixed in about a mole to mole ratio. The long bipolar thick filaments with myosin on the surface can account for the tension development in these molluscan muscles according to the sliding filament theory. These results also suggest that paramyosin may have a specific regulatory role in tension maintenance: in the catch mechanism a phase change in paramyosin may be coupled to the movement of cross-bridges formed between myosin and actin.