Characterization of a calcium-regulation domain of the skeletal-muscle ryanodine receptor

A negatively charged region of the N-terminal portion of the skeletal ryanodine receptor (RyR), located between residues 1872–1923, is involved in Ca ²⁺-dependent regulation of the Ca²⁺-release channel. This region is divergent between the skeletal (RyR1) and cardiac (RyR2) isoforms of the channel, and is known as D3. Ca²⁺ exerts important regulatory functions on the RyR, being involved in both activation and inactivation functions of the channel, i.e. the effects occurring at micromolar and millimolar Ca²⁺ concentrations respectively. To characterize the role of D3 in the Ca²⁺-dependent regulation of the Ca²⁺-release channel, we studied the functional consequences of deleting the D3 region from RyR1 (∆D3-RyR1) using a heterologous expression system, [³H]ryanodine binding assays and single-channel recordings in lipid bilayers. Deletion of the D3 region selectively affected Ca²⁺-dependent regulation of RyR1, but did not alter [³H]ryanodine binding or the effect of other modulators on the RyR. Compared with full-length RyR1 (wt-RyR1), the Ca²⁺-dependence curve of ∆D3-RyR1 is broader, reflecting increased sensitivity to Ca²⁺ activation and decreased sensitivity to Ca²⁺ inactivation. In addition, ∆D3-RyR1 was more resistant to inhibition by Mg²⁺. Comparison of the effect of caffeine on wt-RyR1 and ∆D3-RyR1 suggested that D3 is an important region of RyR that participates in Ca²⁺-dependent activation and inactivation of the Ca²⁺-release channel.