We report the cloning and characterization of DANGER, a novel protein which physiologically binds to inositol 1,4,5-trisphosphate receptors (IP 3R). DANGER is a membrane-associated protein predicted to contain a partial MAB-21 domain. It is expressed in a wide variety of neuronal cell lineages where it localizes to membranes in the cell periphery together with IP3R. DANGER interacts with IP3R in vitro and co-immunoprecipitates with IP3R from cellular preparations. DANGER robustly enhances Ca2+-mediated inhibition of IP3R Ca 2+ release without affecting IP3 binding in microsomal assays and inhibits gating in single-channel recordings of IP3R. DANGER appears to allosterically modulate the sensitivity of IP3R to Ca2+ inhibition, which likely alters IP3R-mediated Ca 2+ dynamics in cells where DANGER and IP3R are co-expressed. © 2006 by The American Society for Biochemistry and Molecular Biology, Inc.