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Summary: Receptor-mediated Ca2+ release from the endoplasmic reticulum (ER) is often followed by Ca2+ entry through Ca2+-release-activated Ca2+ (CRAC) channels in the plasma membrane . RNAi screens have identified STIM1 as the putative ER Ca2+ sensor and CRACM1 (Orai1; ) as the putative store-operated Ca2+ channel. Overexpression of both proteins is required to reconstitute CRAC currents (ICRAC; ). We show here that CRACM1 forms multimeric assemblies that bind STIM1 and that acidic residues in the transmembrane (TM) and extracellular domains of CRACM1 contribute to the ionic selectivity of the CRAC-channel pore. Replacement of the conserved glutamate in position 106 of the first TM domain of CRACM1 with glutamine (E106Q) acts as a dominant-negative protein, and substitution with aspartate (E106D) enhances Na+, Ba2+, and Sr2+ permeation relative to Ca2+. Mutating E190Q in TM3 also affects channel selectivity, suggesting that glutamate residues in both TM1 and TM3 face the lumen of the pore. Furthermore, mutating a putative Ca2+ binding site in the first extracellular loop of CRACM1 (D110/112A) enhances monovalent cation permeation, suggesting that these residues too contribute to the coordination of Ca2+ ions to the pore. Our data provide unequivocal evidence that CRACM1 multimers form the Ca2+-selective CRAC-channel pore. [Copyright &y& Elsevier]