A family of Ca2+ binding proteins (CaBPs) was shown to bind to the inositol 1,4,5-trisphosphate receptor (InsP3R) Ca2+ release channel and gate it in the absence of InsP3, establishing them as protein ligands (Yang et al. (2002) Identification of a family of calcium sensors as protein ligands of inositol trisphosphate receptor Ca2+ release channels. Proc Natl Acad Sci 99(11), 7711-7716) However, the neuronally-restricted expression of CaBP, and its inhibition of InsP3R-mediated Ca2+signaling when over-expressed (Kasri et al. (2004) EMBO J. 23, 312-321; Haynes et al. (2004) J. Biol. Chem. 279, 547-555) have raised questions regarding the functional implications of this regulation. Here we have discovered the Ca2+ binding protein CIB1 (calmyrin) as a ubiquitously-expressed ligand of the InsP3R. CIB1 binds to all mammalian InsP3R isoforms in a Ca2+-sensitive manner dependent on its two functional EF hands, and activates InsP3R channel gating in the absence of InsP3. In contrast, over-expression of CIB1 or CaBP1 attenuated InsP3R-dependent Ca2+ signaling, and in vitro pre-exposure to CIB1 reduced the number of channels available for subsequent stimulation by InsP3. These results establish CIB1 as a ubiquitously-expressed activating and inhibiting protein ligand of the InsP3R.