ER-luminal [Ca²⁺] regulation of InsP₃ receptor gating mediated by an ER-luminal peripheral Ca²⁺-binding protein

Modulating cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i) by endoplasmic reticulum (ER)-localized inositol 1,4,5-trisphosphate receptor (InsP₃R) Ca²⁺-release channels is a universal signaling pathway that regulates numerous cell-physiological processes. Whereas much is known regarding regulation of InsP₃R activity by cytoplasmic ligands and processes, its regulation by ER-luminal Ca²⁺ concentration ([Ca²⁺]_ER) is poorly understood and controversial. We discovered that the InsP₃R is regulated by a peripheral membrane-associated ER-luminal protein that strongly inhibits the channel in the presence of high, physiological [Ca²⁺]_ER. The widely-expressed Ca²⁺-binding protein annexin A1 (ANXA1) is present in the nuclear envelope lumen and, through interaction with a luminal region of the channel, can modify high-[Ca²⁺]_ER inhibition of InsP₃R activity. Genetic knockdown of ANXA1 expression enhanced global and local elementary InsP₃-mediated Ca²⁺ signaling events. Thus, [Ca²⁺]_ER is a major regulator of InsP₃R channel activity and InsP₃R-mediated [Ca²⁺]_i signaling in cells by controlling an interaction of the channel with a peripheral membrane-associated Ca²⁺-binding protein, likely ANXA1.