Store-operated channels (SOC) are Ca2+-permeable channels that are activated by IP3-receptor-mediated Ca2+ depletion of the endoplasmic reticulum (ER). Recent studies identify a membrane pore subunits, Orail and a Ca2+ sensor on ER, STIM1 as components of Ca2+ release-activated Ca2+ (CRAC) channels, which are well-characterized SOCs. On the other hand, proteins that act as modulators of SOC activity remain to be identified. Calumin is a Ca2+-binding protein that resides on the ER and functional experiments using calumin-null mice demonstrate that it is involved in SOC function, although its role is unknown. This study used electrophysiological analysis to explore whether calumin modulates CRAC channel activity. CRAC channel currents were absent in HEK293 cells co-expressing calumin with the CRAC channel components, Orai1 or STIM1. Meanwhile, HEK cells that co-expressed calumin with CRAC channels exhibited larger currents with slower inactivation than cells expressing CRAC channels alone. The current-voltage relationship showed an inwardly rectifying current, but a negative shift in the reversal potential of greater than 60 mV was observed in HEK cells co-expressing calumin with CRAC channels. In addition, the permeability coefficient ratio of Ca2+ over monovalent cations was much lower than that of cells expressing CRAC channels alone. Replacement of Na+ with N-methyl-D-glucamine(+) in the external solution noticeably diminished the CRAC current in HEK cells co-expressing calumin and CRAC channels. In a Cs+-based external solution, CRAC current was not observed in either cell-type. In addition, Ca2+ imaging analysis revealed that co-transfection of calumin reduced extracellular Ca2+ influx via CRAC channels. Further, calumin was shown to be directly associated with CRAC channels. These results reveal a novel mechanism for the regulation of CRAC channels by calumin. (C) 2011 Elsevier Inc. All rights reserved.