Type-specific proteins of single IIM fibres from cat muscle.

The fibre type IIM, present in the jaw-closer muscles of the cat, has a characteristic form of myosin. To analyse other proteins present in IIM fibres and the usual types I, IIA and IIB, 14C-labelled typed single fibres were analysed by 2-dimensional gel electrophoresis. Whereas types I, IIA and IIB contained both alpha- and beta-forms of tropomyosin, IIM fibres contained only a single form, shown by 1-dimensional peptide mapping to be different from the tropomyosins present in the other fibre types. This single form of tropomyosin, and two soluble proteins seen only in IIM fibres, were also found in IIM fibre-containing muscles from other species, demonstrating that IIM fibres are a distinct type.