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 Ca2+-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 (Delta D3-RyR1) using a heterologous expression system, [H-3]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 [H-3]ryanodine binding or the effect of other modulators on the RyR. Compared with full-length RyR1 (wt-RyR1), the Ca2+ dependence curve of Delta D3-RyR1 is broader, reflecting increased sensitivity to Ca2+ activation and decreased sensitivity to Ca2+ inactivation. In addition, Delta D3-RyR1 was more resistant to inhibition by Mg2+. Comparison of the effect of caffeine on wt-RyR1 and Delta D3-RyR1 suggested that D3 is an important region of RyR that participates in Ca2+-dependent activation and inactivation of the Ca2+-release channel.