Role of cardiac ryanodine receptor calmodulin‐binding domains in mediating the action of arrhythmogenic calmodulin N‐domain mutation N54I

The Ca2+‐sensing protein calmodulin (CaM) inhibits cardiac ryanodine receptor (RyR2)‐mediated Ca2+ release. CaM mutations associated with arrhythmias and sudden cardiac death have been shown to diminish CaM‐dependent inhibition of RyR2, but the underlying mechanisms are not well understood. Nearly all arrhythmogenic CaM mutations identified are located in the C‐domain of CaM and exert marked effects on Ca2+ binding to CaM and on the CaM C‐domain interaction with the CaM‐binding domain 2 (CaMBD2) in RyR2. Interestingly, the arrhythmogenic N‐domain mutation CaM‐N54I has little or no effect on Ca2+ binding to CaM or the CaM C‐domain‐RyR2 CaMBD2 interaction, unlike all CaM C‐domain mutations. This suggests that CaM‐N54I may diminish CaM‐dependent RyR2 inhibition by affecting CaM N‐domain interactions with RyR2 CaMBDs other than CaMBD2. To explore this possibility, we assessed the effects of deleting each of the four known CaMBDs in RyR2 (CaMBD1a, ‐1b, ‐2, or ‐3) on the CaM‐dependent inhibition of RyR2‐mediated Ca2+ release in HEK293 cells. We found that removing CaMBD1a, CaMBD1b, or CaMBD3 did not alter the effects of CaM‐N54I or CaM‐WT on RyR2 inhibition. On the other hand, deleting RyR2‐CaMBD2 abolished the effects of both CaM‐N54I and CaM‐WT. Our results support that CaM‐N54I causes aberrant RyR2 regulation via an uncharacterized CaMBD or less likely CaMBD2, and that RyR2 CaMBD2 is required for the actions of both N‐ and C‐domain CaM mutations. Moreover, our results show that CaMBD1a is central to RyR2 regulation, but CaMBD1a, CaMBD1b, and CaMBD3 are not required for CaM‐dependent inhibition of RyR2 in HEK293 cells. The Asn‐54‐Ile mutation in calmodulin (CaM) diminishes inhibition of the cardiac sarcoplasmic reticulum (SR) Ca2+ release channel (RyR2), but the underlying mechanism remains unknown. Using CaM‐N54I, we investigate the four reported CaM‐binding domains (CaMBDs) in RyR2 for their role in CaM‐dependent RyR2 inhibition. We find that three putative CaMBDs are not strictly required for native CaM‐dependent RyR2 inhibition, and that effects of CaM‐N54I are likely mediated via an uncharacterized CaMBD.