Skeletal (RyR1) and cardiac muscle (RyR2) isoforms of ryanodine receptor calcium 
channels are inhibited by millimollar Ca(2+), but the affinity of RyR2 for 
inhibitory Ca(2+) is ~10 times lower than that of RyR1. Previous studies 
demonstrated that the C-terminal quarter of RyR has critical domain(s) for 
Ca(2+) inactivation. To obtain further insights into the molecular basis of 
regulation of RyRs by Ca(2+), we constructed and expressed 18 RyR1-RyR2 chimeras 
in HEK293 cells and determined the Ca(2+) activation and inactivation affinities 
of these channels using the [(3)H]ryanodine binding assay. Replacing two 
distinct regions of RyR1 with corresponding RyR2 sequences reduced the affinity 
for Ca(2+) inactivation. The first region (RyR2 amino acids 4020-4250) contains 
two EF-hand Ca(2+) binding motifs (EF1, amino acids 4036-4047; EF2, amino acids 
4071-4082), and the second region includes the putative second transmembrane 
segment (S2). A RyR1-backbone chimera containing only EF2 from RyR2 had a modest 
(not significant) change in Ca(2+) inactivation, whereas another chimera channel 
carrying only EF1 from RyR2 had a significantly reduced level of Ca(2+) 
inactivation. The results suggest that EF1 is a more critical determinant for 
RyR inactivation by Ca(2+). In addition, activities of the chimera carrying RyR2 
EF-hands were suppressed at 10-100 μM Ca(2+), and the suppression was relieved 
by 1 mM Mg(2+). The same effects have been observed with wild-type RyR2. A 
mutant RyR1 carrying both regions replaced with RyR2 sequences (amino acids 
4020-4250 and 4560-4618) showed a Ca(2+) inactivation affinity comparable to 
that of RyR2, indicating that these regions are sufficient to confer RyR2-type 
Ca(2+)-dependent inactivation on RyR1.