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 sarcoplasmic 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 tryptic fragments of the purified protein. The other group of cDNAs encodes 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 identity is present between alpha- and beta-isoforms of the bullfrog. The primary structure of the alpha-isoform is highly (80%) homologous to the ryanodine-binding protein cloned from rabbit skeletal muscle (type 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% overall identity with type 1. Analyses of RNA from various tissues of the bullfrog demonstrate that the beta-isoform is widely expressed, while the 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 including skeletal muscle excitation-contraction coupling were discussed.