3 RYANODINE RECEPTOR ISOFORMS EXIST IN AVIAN STRIATED MUSCLES

Two isoforms of the ryanodine receptor (termed alpha and beta) are coe xpressed in avian fast twitch skeletal muscle, whereas a single isofor m is expressed in avian cardiac muscle. We have investigated the relat ionship between these three proteins, comparing several different prop erties. First, the three receptor isoform subunits have different mobi lities on SDS-polyacrylamide gels. Second, monoclonal antibodies again st the chicken skeletal muscle receptor isoforms recognize shared and unique epitopes in each receptor protein, indicating there is not a si mple antigenic relationship between the isoforms. Third, the three rec eptor isoforms exhibit different susceptibilities to proteolysis by tr ypsin, and limited tryptic digestion yields a different peptide map fo r each isoform. Fourth, in native sarcoplasmic reticulum membranes, th e chicken muscle receptor isoforms are phosphorylated to different ext ents by the multifunctional calcium/calmodulin-dependent protein kinas e 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, pr oduced against a synthetic peptide containing the site phosphorylated by this kinase in the mammalian cardiac muscle receptor, by immunoprec ipitation showed markedly different avidities for the receptor isoform s, 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). Thes e results indicate that three distinct ryanodine receptor proteins are expressed in chicken striated muscles.