Isolation of a Second Form of Calsequestrin

In the sarcoplasmic reticulum of individual rabbits, calsequestrin usually is observed as a single peptide in sodium dodecyl sulfate-polyacrylamide gels (Form 1). In some rabbits calsequestrin (Form 2) is found with a molecular weight 6% less than that of Form 1. In still other rabbits the two forms coexist in equal amounts. Form 2 is not a degradation product of Form 1 since mixing of muscle types before homogenization does not lead to breakdown of Form 1. Sarcoplasmic reticulum containing Form 2 calsequestrin is increased in its capacity for binding and transport of Ca2+. Form 2 calsequestrin has been highly purified and has Ca2+-binding capacity equal to Form 1. Amino acid analyses of Forms 1 and 2 indicate close similarity with differences in the content of tyrosine, cysteine, and methionine. Peptide maps of chymotryptic digests also show similarity but indicate two differences in peptide classes. Chicken antibodies against rabbit calsequestrin Form 1 also precipitate calsequestrin Form 2 and precipitin lines in Ouchterlony plates are confluent. Initial genetic studies indicate that inheritance of Form 2 calsequestrin is complex.