CLONING AND CHARACTERIZATION OF FIBER TYPE-SPECIFIC RYANODINE RECEPTOR ISOFORMS IN SKELETAL-MUSCLES OF FISH

We have cloned a group of cDNAs that encodes the skeletal ryanodine re ceptor isoform (RyR1) of fish from a blue marlin extraocular muscle li brary. The cDNAs encode a protein of 5,081 amino acids with a calculat ed molecular mass of 576,302 Da. The deduced amino acid sequence shows strong sequence identity to previously characterized RyR1 isoforms. A n RNA probe derived from a clone of the full-length marlin RyR1 isofor m hybridizes to RNA preparations from extraocular muscle and slow-twit ch skeletal muscle but not to RNA preparations from fast-twitch skelet al or cardiac muscle. We have also isolated a partial RyR clone from m arlin and toadfish fast-twitch muscles that shares 80% sequence identi ty with the corresponding region of the full-length RyR1 isoform, and a RNA probe derived from this clone hybridizes to RNA preparations fro m fast-twitch muscle but not to slow-twitch muscle preparations. Weste rn blot analysis of slow-twitch muscles in fish indicates the presence of only a single high-molecular-mass RyR protein corresponding to RyR 1. [H-3]ryanodine binding assays revealed the fish slow-twitch muscle RyR1 had a greater sensitivity for Ca2+ than the fast-twitch muscle Ry R1. The results indicate that, in fish muscle, fiber type-specific RyR 1 isoforms are expressed and the two proteins are physiologically dist inct.