A new myofibrillar protein characteristic of type I human skeletal muscle fibres.

Analysis of a single human type I (slow-twitch) skeletal muscle fibres by two-dimensional gel electrophoresis shows that they contain a specific protein of subunit Mr 34 x 10(3) and pI 6.0, 6.1 and 6.2 (three spots) which is absent from type IIA and IIB muscle fibres. This type I protein is located in the I-band of isolated myofibrils as demonstrated by the indirect immunofluorescence using specific antisera against the denatured form of type I protein. Type I protein could only be extracted from myofibrils with high salt solutions (0.6-1 M KCl), conditions which also coextract actin and myosin, demonstrating its firm attachment. Rat and cat type I muscle fibres as well as the slow-tonic fibres from chicken anterior latissimus dorsi also contained a protein homologous to the human type I protein. The human type I protein was found to be different from the known regulatory and contractile muscle proteins by electrophoretic and immunological criteria, and may therefore be a new myofibrillar protein in type I muscle fibres.