Roles of the NH2-terminal Domains of Cardiac Ryanodine Receptor in Ca2+ Release Activation and Termination [Molecular Bases of Disease]

January 27th, 2015 by Liu, Y., Sun, B., Xiao, Z., Wang, R., Guo, W., Zhang, J. Z., Mi, T., Wang, Y., Jones, P. P., Petegem, F. V., Chen, S. R. W.

The NH2-terminal region (residues 1-543) of the cardiac ryanodine receptor (RyR2) harbors a large number of mutations associated with cardiac arrhythmias and cardiomyopathies. Functional studies have revealed that the NH2-terminal region is involved in the activation and termination of Ca2+ release. The three dimensional structure of the NH2-terminal region has recently been solved. It is composed of three domains (A, B, and C). However, the roles of these individual domains in Ca2+ release activation and termination are largely unknown. To understand the functional significance of each of these NH2-terminal domains, we systematically deleted these domains and assessed their impact on caffeine- or Ca2+-induced Ca2+ release and store overload induced Ca2+ release (SOICR) in HEK293 cells. We found that all deletion mutants are capable of forming caffeine and ryanodine sensitive functional channels, indicating that the NH2-terminal region is not essential for channel gating. Ca2+ release measurements revealed that deleting domain A markedly reduced the threshold for SOICR termination, but had no effect on caffeine or Ca2+ activation or the threshold for SOICR activation, whereas deleting domain B substantially enhanced caffeine and Ca2+ activation and lowered the threshold for SOICR activation and termination. On the other hand, deleting domain C suppressed caffeine activation, abolished Ca2+ activation and SOICR, and diminished protein expression. These results suggest that domain A is involved in channel termination, domain B is involved in channel suppression, and domain C is critical for channel activation and expression. Our data shed new insights into the structure-function relationship of the NH2-terminal domains of RyR2 and the action of NH2-terminal disease mutations.