Positioning of major tryptic fragments in the Ca2+ release channel (ryanodine receptor) resulting from partial digestion of rabbit skeletal muscle sarcoplasmic reticulum.

Site-specific antibodies against different regions of the Ca2+ release channel of skeletal muscle sarcoplasmic reticulum (ryanodine receptor) were developed and used as probes for immunoblotting of the major tryptic fragments resulting from partial digestion of the ryanodine receptor in sarcoplasmic reticulum membranes. Five major tryptic fragments, some of which migrated as doublets, with apparent masses of 150/140, 110/100, 55, 170/160, and 76 kDa were ordered so that they covered the bulk of the protein from the NH2 to the COOH terminus. Tryptic subfragments of 53, 63, and 115/95 kDa were also derived from the 150/140-, 110/100-, and 170/160-kDa fragments, respectively. All of these fragments and subfragments were detected only in the insoluble membrane fraction of the trypsinized sarcoplasmic reticulum. Upon Na2CO3 extraction, the 150/140-, 110/100-, and 55-kDa fragments could be solubilized, suggesting their origin in the cytoplasmic domain of the ryanodine receptor. The 170/160- and 76-kDa fragments and the 115/95-kDa subfragment remained insoluble, suggesting their origin in the transmembrane region of the ryanodine receptor. The 150/140-, 110/100-, 170/160-, and 76-kDa fragments and the 115/95 subfragment co-migrated near the bottom of a sucrose density gradient after CHAPS solubilization, suggesting that they were associated in an oligomeric complex. By contrast, the 53- and 63-kDa subfragments and the 55-kDa fragment were detected near the top of the sucrose gradient after CHAPS solubilization, suggesting that they were not involved in the formation of the core of the oligomeric complex. These studies identify 7 sites that are exposed to trypsin in the ryanodine receptor in sarcoplasmic reticulum, 3 of which are novel and 4 of which are in the same location as proteolytic cleavage sites identified previously (Marks, A. R., Fleischer, S., and Tempst, P. (1990) J. Biol. Chem. 265, 13143-13149).