Mutation of Divergent Region 1 Alters Caffeine and Ca2+ Sensitivity of the Skeletal Muscle Ca2+Release Channel (Ryanodine Receptor)

Replacement of amino acids 4187-4628 in the skeletal muscle Ca(2+) release channel (skeletal ryanodine receptor (RyR1)), including nearly all of divergent region 1 (amino acids 4254-4631), with the corresponding cardiac ryanodine receptor (RyR2) sequence leads to increased sensitivity of channel activation by caffeine and Ca(2+) and to decreased sensitivity of channel inactivation by elevated Ca(2+) (Du, G. G., and MacLennan, D. H. (1999) J. Biol. Chem. 274, 26120-26126). In further investigations, this region was subdivided by the construction of new chimeras, and alterations in channel function were detected by measurement of the caffeine dependence of in vivo Ca(2+) release and the Ca(2+) dependence of (3)Hryanodine binding. Chimera RF10a (amino acids 4187-4381) had a lower EC(50) value for activation by caffeine, and RF10c (4557-4628) had a higher EC(50) value, whereas the EC(50) value for chimera RF10b (4382-4556) was unchanged. Chimeras RF10b and RF10c were more sensitive to activation by Ca(2+), whereas RF10a was less sensitive to inactivation by Ca(2+), implicating RF10b and RF10c in Ca(2+) activation and RF10a in Ca(2+) inactivation. Deletion of much of divergent region 1 sequence to create mutant Delta4274-4535 led to higher caffeine and Ca(2+) sensitivity of channel activation and to lower Ca(2+) sensitivity for inactivation. Thus, deletion results demonstrate that caffeine, Ca(2+), and ryanodine binding sites are not located in amino acids 4274-4535. Nevertheless, the properties of the deletion and chimeric mutants demonstrate that amino acids 4274-4535 and three shorter sequences in this region (F10a, amino acids 4187-4381; F10b, 4382-4556; and F10c, 4557-4628) in RyR1 modulate Ca(2+) and caffeine sensitivity of the Ca(2+) release channel.