PRIMARY STRUCTURE AND DISTRIBUTION OF RYANODINE BINDING-PROTEIN ISOFORMS OF THE BULLFROG SKELETAL-MUSCLE

We have cloned two groups of cDNAs which encode isoforms of ryanodine- binding protein/Ca2+ release channel of the bullfrog skeletal muscle s arcoplasmic reticulum. One of the cDNA groups encodes the protein of 5 ,037 (or 5,031 with a deletion) amino acids with a molecular weight of 571,262 (or 570,607), which is identified as the alpha-isoform of the ryanodine-binding protein based on the amino acid sequence of three t ryptic fragments of the purified protein. The other group of cDNAs enc odes the protein of 4,868 amino acids with molecular weight of 553,029 , which contains the sequences of three proteolytic fragments derived from the beta-isoform protein. About 70% of the amino acid sequence id entity is present between alpha- and beta-isoforms of the bullfrog. Th e primary structure of the alpha-isoform is highly (80%) homologous to the ryanodine-binding protein cloned from rabbit skeletal muscle (typ e 1). The beta-isoform, on the other hand, is more than 85% identical with that from the rabbit brain (type 3), while it has only 67% overal l identity with type 1. Analyses of RNA from various tissues of the bu llfrog demonstrate that the beta-isoform is widely expressed, while th e alpha-isoform is expressed mainly in skeletal muscle. A phylogenetic analysis of the ryanodine binding protein/Ca2+ release channel family suggests that the various types of Ca2+ release channels have evolved from an ancestor gene. Possible differential roles of alpha- and beta -isoforms of ryanodine-binding protein in Ca2+ release mechanisms incl uding skeletal muscle excitation contraction coupling were discussed.