Fast-twitch and slow-twitch skeletal muscles express the same isoform of calreticulin

We have characterized calreticulin protein and mRNA in fast-twitch and slow-twitch skeletal muscle. SR vesicles isolated from fast-twitch and slow-twitch skeletal muscles contain calreticulin protein immunoreactive with anti-calreticulin antibody. In addition, the fast-twitch skeletal muscle form of calreticulin is shown to be identical to slow-twitch form of the protein based on the identity of cDNA clones encoding fast-twitch and slow-twitch forms of the protein and hybridization of a fast-twitch or slow-twitch calreticulin cDNA probe to the same mRNA species. Based on these observations we conclude that fast-twitch and slow-twitch skeletal muscles express the same form of calreticulin.