Characterization and fibre type distribution of a new myofibrillar protein of molecular weight 32 kDa.

A new basic protein of molecular weight 32 kDa has been isolated and purified to homogeneity from skeletal muscles rich in type I fibres. By the use of a specific monoclonal antibody, the protein has been shown to be present in all type I fibres and some type II fibres, the number of which varies with the muscle and the region of the muscle sectioned. A protein of similar properties could not be isolated from rabbit muscles consisting predominantly of type II fibres. By fluorescence microscopy, the protein has been shown to be located in the Z-disc from which the presence of divalent cations, probably calcium, facilitates its extraction at low ionic strength. The protein is unusual in that its distribution does not correlate completely with the known muscle fibre types and in that as yet there is no evidence for the presence of an isoform in those cells that do not stain with the specific antibody for the 32 kDa protein isolated from slow muscles.