What is it about?
Inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) are ubiquitously expressed calcium (Ca2+) release channels which are principally located on the endoplasmic reticulum (ER). Following binding of both IP3 and Ca2+, IP3Rs mediate explosive Ca2+ release from the ER to cytoplasm. This Ca2+ release plays fundamental roles in a multitude of events as diverse as muscle contraction, gene transcription, secretion and cell fate decisions and is often altered in disease states. In the presence of IP3, increasing intracellular Ca2+ concentration [Ca2+]i initially greatly augments IP3R channel opening. However, increasing [Ca2+]i further, decreases channel activity to result in a characteristic “bell-shaped” regulation of IP3R channel activity by Ca2+. A long-standing question has been what structural motifs in IP3R confer this complex dependency of Ca2+. Based on recently resolved near atomic resolution cryo-EM structures of IP3Rs and ryanodine receptors (RyRs), we investigated the functional consequences of substituting evolutionarily well-conserved putative Ca2+ coordinating acidic amino acid residues on IP3R channel activation. We experimentally confirmed, these substitutions do not compromise the ability of the channels to properly localize to the ER or form complexes. Intriguingly, neutralizing the negative charge on these conserved residues dramatically diminished (1) IP3 generating agonist-evoked Ca2+ release, (2) fundamental Ca2+ signals mediated by IP3Rs, (3) single channel open-probability and (4) shifted Ca2+ dependency of IP3R channels to a higher Ca2+ concentrations. These results indicate Ca2+ binding to this binding pocket, coordinates Ca2+ and is essential for appropriate channel activation and subsequent pore opening in response to optimal physiological stimuli.
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Why is it important?
Our investigations identified a well-conserved Ca2+ binding pocket in IP3Rs. In addition to binding IP3, Ca2+ binding to this pocket is also an absolute pre-requisite to facilitate channel opening. Likewise, a similar Ca2+ binding pocket exists in RyRs and it was previously shown to be critical for channel activation. Overall, our results indicate that a common underlying mechanism underpins activation of these two major classes of intracellular calcium release channels.
Perspectives
Bi-phasic regulation of IP3Rs by Ca2+ was first identified in 1990s and later successfully confirmed by various groups across the world. Working on this exciting open research question to identify the Ca2+ binding pocket in IP3Rs was a tremendous learning experience for all of us. I thank all the co-authors, Yule laboratory members and collaborators for being valuable scientific resources and for their constant support. We are delighted our findings are now published in PNAS! We hope you enjoy reading this article as much as we enjoyed working on it.
David Yule
University of Rochester
Read the Original
This page is a summary of: Functional determination of calcium-binding sites required for the activation of inositol 1,4,5-trisphosphate receptors, Proceedings of the National Academy of Sciences, September 2022, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2209267119.
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