The high-affinity calcium–calmodulin-binding site does not play a role in the modulation of type 1 inositol 1,4,5-trisphosphate receptor function by calcium and calmodulin

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Abstract
Modulation of the inositol 1,4,5-trisphosphate (InsP3) receptors (InsP3R) by cytosolic calcium (Ca2+) plays an essential role in Ca2+ signalling, but structural determinants and mechanisms responsible for the InsP3R regulation by Ca2+ are poorly understood. In the present study, we expressed rat InsP3R type 1 (InsP3R1) in Spodoptera frugiperda cells using a baculovirus-expression system and reconstituted the recombinant InsP3R1 into planar lipid bilayers for functional analysis. We observed only minor effects of 0.5mM of calmodulin (CaM) antagonist W-7 on the Ca2+ dependence of InsP3R1. Based on a previous analysis of mouse InsP3R1 [Yamada, Miyawaki, Saito, Nakajima, Yamamoto-Hino, Ryo, Furuichi and Mikoshiba (1995) Biochem J. 308, 83–88], we generated the Trp1577→Ala (W1577A) mutant of rat InsP3R1 which lacks the high-affinity Ca2+—CaM-binding site. We found that the W1577A mutant displayed a bell-shaped Ca2+ dependence similar to the wild-type InsP3R1 in planar lipid bilayers. Activation of B cell receptors resulted in identical Ca2+ signals in intact DT40 cells lacking the endogenous InsP3R and transfected with the wild-type InsP3R1 or the W1577A mutant cDNA subcloned into a mammalian expression vector. In the planar lipid bilayer experiments, we showed that both wild-type InsP3R1 and W1577A mutant were equally sensitive to inhibition by exogenous CaM. From these results, we concluded that the interaction of CaM with the high-affinity Ca2+—CaM-binding site in the coupling domain of the InsP3R1 does not play a direct role in biphasic modulation of InsP3R1 by cytosolic Ca2+ or in InsP3R1 inhibition by CaM.