Molecular identification of the ryanodine receptor Ca2+ sensor.

We have investigated the molecular basis for ryanodine receptor (RyR) activation by Ca2+ by using site-directed mutagenesis together with functional assays consisting of Ca2+ release measurements and single channel recordings in planar lipid bilayers. We report here that a single substitution of alanine for glutamate at position 3885 (located in the putative transmembrane sequence M2 of the type 3 RyR) reduces the Ca2+ sensitivity, as measured by single channel activation, by more than 10,000-fold, without apparent changes in channel conductance and in modulation by other ligands (e.g. ATP and ryanodine). Co-expression of the wild type and mutant RyR proteins results in the synthesis of single channels that have intermediate Ca2+ sensitivities. These results suggest that the glutamates at position 3885 of each monomer may act in a coordinated way to form the Ca2+ sensor in the tetrameric structure corresponding to RyR.