3 RYANODINE RECEPTOR ISOFORMS EXIST IN AVIAN STRIATED MUSCLES

Two isoforms of the ryanodine receptor (termed alpha and beta) are coexpressed in avian fast twitch skeletal muscle, whereas a single isoform is expressed in avian cardiac muscle. We have investigated the relationship between these three proteins, comparing several different properties. First, the three receptor isoform subunits have different mobilities on SDS-polyacrylamide gels. Second, monoclonal antibodies against the chicken skeletal muscle receptor isoforms recognize shared and unique epitopes in each receptor protein, indicating there is not a simple antigenic relationship between the isoforms. Third, the three receptor isoforms exhibit different susceptibilities to proteolysis by trypsin, and limited tryptic digestion yields a different peptide map for each isoform. Fourth, in native sarcoplasmic reticulum membranes, the chicken muscle receptor isoforms are phosphorylated to different extents by the multifunctional calcium/calmodulin-dependent protein kinase II (beta > cardiac > alpha). Fifth, the sites phosphorylated by the calcium/calmodulin-dependent protein kinase in the chicken cardiac and skeletal receptor isoforms are not equivalent. A polyclonal serum, produced against a synthetic peptide containing the site phosphorylated by this kinase in the mammalian cardiac muscle receptor, by immunoprecipitation showed markedly different avidities for the receptor isoforms, and recognized only the cardiac receptor isoform on Western blots. Sixth, the chicken ryanodine receptor isoforms differ in the extent to which they bind azido[I-125]calmodulin (alpha > beta > cardiac). These results indicate that three distinct ryanodine receptor proteins are expressed in chicken striated muscles.