The fast-twitch muscle calsequestrin isoform predominates in rabbit slow-twitch soleus muscle.

The major form of calsequestrin in rabbit slow-twitch soleus muscle is shown to be identical to that isolated and cloned from rabbit fast-twitch muscle on the following bases: identity of cDNAs cloned from mRNAs from the two muscle sources; equivalent hybridization of a fast-twitch calsequestrin cDNA probe to mRNAs isolated from fast-twitch and slow-twitch muscles; identity of the 23 amino-terminal amino acids; strong binding of 45Ca2+ in a gel overlay of slow muscle sarcoplasmic reticulum protein to a band at the level of the fast-twitch calsequestrin isoform and only weak binding at the level of the cardiac isoform. No evidence was obtained for developmentally regulated alternative splicing of the calsequestrin transcript in mature slow or fast-twitch muscle.