Ca²⁺-sensor region of IP₃ receptor controls intracellular Ca²⁺ signaling

Many important cell functions are controlled by Ca²⁺ release from intracellular stores via the inositol 1,4,5-trisphosphate receptor (IP₃R), which requires both IP₃ and Ca²⁺ for its activity. Due to the Ca²⁺ requirement, the IP₃R and the cytoplasmic Ca²⁺ concentration form a positive feedback loop, which has been assumed to confer regenerativity on the IP₃-induced Ca²⁺ release and to play an important role in the generation of spatiotemporal patterns of Ca²⁺ signals such as Ca²⁺ waves and oscillations. Here we show that glutamate 2100 of rat type 1 IP₃R (IP₃R1) is a key residue for the Ca²⁺ requirement. Substitution of this residue by aspartate (E2100D) results in a 10-fold decrease in the Ca²⁺ sensitivity without other effects on the properties of the IP₃R1. Agonist-induced Ca²⁺ responses are greatly diminished in cells expressing the E2100D mutant IP₃R1, particularly the rate of rise of initial Ca²⁺ spike is markedly reduced and the subsequent Ca²⁺ oscillations are abolished. These results demonstrate that the Ca²⁺ sensitivity of the IP₃R is functionally indispensable for the determination of Ca²⁺ signaling patterns.