Conformational landscape and clustering of human type 2 IP 3 receptor in lipid nanodiscs

ABSTRACT Inositol 1,4,5-trisphosphate (IP 3 ) receptors (IP 3 Rs) are tetrameric ER Ca 2+ channels that shape intracellular Ca 2+ signaling in response to IP 3 , regulating many physiological processes. The structural basis for subtype-specific regulation of three subtypes (IP 3 R-1-3) remains incompletely understood, due to the lack of IP 3 R-2 structures. Here, we determined cryo-electron microscopy (cryo-EM) structures of human IP 3 R-2 in distinct conformations with and without IP 3 , Ca 2+ , and ATP. These structures define the conformational ensembles adopted by IP 3 R-2 and delineate ligand-binding interactions. Comparison with rat IP 3 R-1 and human IP 3 R-3 highlights shared architectural features and isoform-specific differences that underlie subtype-specific functional properties. We also resolved structures of IP 3 R-2 clusters, providing insight into mechanisms of ligand-dependent clustering. Together, these findings establish a structural framework for human IP 3 R-2, linking ligand recognition to conformational transitions and interchannel interfaces, and illuminate how subtype-specific features and clustering may shape cellular Ca 2+ signaling.