Ca2+ influx through Ca2+-release activated Ca2+ channels (CRAC) is critical for activating immune cells. Oral and STIM proteins comprise the molecular components of CRAC. We previously observed that curcumin and caffeic acid phenethyl ester (CAPE) inhibit CRAC current in Orai1/STIM1-co-expressing HEK293 cells (Nam et al., 2009: Shin et al., 2011) [1,2]. Both compounds contain electrophilic alpha,beta-unsaturated carbonyl groups that potentially form Michael addition with cysteine residues. We investigated the sensitivity of cysteine mutated Orai1 to curcumin and CAPE to delineate their inhibitory mechanism. Replacing the 195 cysteine residue with serine (C195S) reversed the effect of CAPE from inhibition to facilitation and significantly weakened the inhibitory effect of curcumin. Tetrahydrocurcumin, a curcumin metabolite, showed a less potent inhibitory effect on I-CRAC, and this effect was abolished in C195S Orai1. Additive mutation of other cysteines (C143S and C126S) had no further influence on the effects of CAPE and curcumin. These results indicate that the electrophilic addition to the Orai1 195Cys was responsible for the inhibitory effect of I-CRAC by curcumin and CAPE. (C) 2012 Elsevier Inc. All rights reserved.