Ligand sensitivity of type-1 inositol 1,4,5-trisphosphate receptor is enhanced by the D2594K mutation

Inositol 1,4,5-trisphosphate receptor (IP(3)R) and ryanodine receptor (RyR) are homologous cation channels that mediate release of Ca(2+) from the endoplasmic/sarcoplasmic reticulum (ER/SR) and thereby are involved in many physiological processes. In previous studies, we determined that when the D2594 residue, located at or near the gate of the IP(3)R type 1, was replaced by lysine (D2594K), a gain of function was obtained. This mutant phenotype was characterized by increased IP(3) sensitivity. We hypothesized the IP(3)R1-D2594 determines the ligand sensitivity of the channel by electrostatically affecting the stability of the closed and open states. To test this possibility, the relationship between the D2594 site and IP(3)R1 regulation by IP(3), cytosolic, and luminal Ca(2+) was determined at the cellular, subcellular, and single-channel levels using fluorescence Ca(2+) imaging and single-channel reconstitution. We found that in cells, D2594K mutation enhances the IP(3) ligand sensitivity. Single-channel IP(3)R1 studies revealed that the conductance of IP(3)R1-WT and -D2594K channels is similar. However, IP(3)R1-D2594K channels exhibit higher IP(3) sensitivity, with substantially greater efficacy. In addition, like its wild type (WT) counterpart, IP(3)R1-D2594K showed a bell-shape cytosolic Ca(2+)-dependency, but D2594K had greater activity at each tested cytosolic free Ca(2+) concentration. The IP(3)R1-D2594K also had altered luminal Ca(2+) sensitivity. Unlike IP(3)R1-WT, D2594K channel activity did not decrease at low luminal Ca(2+) levels. Taken together, our functional studies indicate that the substitution of a negatively charged residue by a positive one at the channels’ pore cytosolic exit affects the channel’s gating behavior thereby explaining the enhanced ligand-channel’s sensitivity. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00424-023-02796-x.