Ultrastructural studies on paramyosin core filaments from native thick filaments in catch muscles.

Catch muscles of molluscs usually have thick filaments of about 100 nm in diameter. The filament is constructed of a paramyosin core filament and an outer layer of myosin molecules. Myosin molecules are situated on the paramyosin core filament towards both ends, and they consequently have polarity towards both ends. According to our observations on the paramyosin core filaments from the native thick filaments, they bear regular cross-striations on their surfaces of about 14.5 nm periodicity when incubated in a KCl solution for a short time. The periodic pattern is supposed to be representative of peculiar arrangements of paramyosin molecules in the core, but the periodic pattern disappeared during incubation in a solution of high concentration KCl for a prolonged time, and a 'Bear-Selby net' pattern appeared substitutionally. These 'Bear-Selby net' patterns were conveniently divided into 3 types among 4 paramyosin core filaments from 4 'catch' muscles; the adductors of a pecten, an oyster and a clam, and the byssus retractor of a mussel. The 'Bear-Selby net' of an oyster resembled that of a pecten. Purified paramyosin crystals from the 4 muscles showed a common periodicity of about 72.5 nm. Electrophoresis with SDS of the 4 paramyosins on 6% polyacrylamide gels revealed molecular weights at 104 kD from a pecten, 105 kD from an oyster, 103 kD from a clam and 105 kD from a mussel.