title: Calumin, a Ca²⁺-binding protein on the endoplasmic reticulum, alters the ion permeability of Ca²⁺ release-activated Ca²⁺ (CRAC) channels.
abstract: Store-operated channels (SOC) are Ca(2+)-permeable channels that are activated by IP(3)-receptor-mediated Ca(2+) depletion of the endoplasmic reticulum (ER). Recent studies identify a membrane pore subunits, Orai1 and a Ca(2+) sensor on ER, STIM1 as components of Ca(2+) release-activated Ca(2+) (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 Ca(2+)-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 60mV was observed in HEK cells co-expressing calumin with CRAC channels. In addition, the permeability coefficient ratio of Ca(2+) 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, Ca(2+) imaging analysis revealed that co-transfection of calumin reduced extracellular Ca(2+) 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.
authors.affiliation: Department of Molecular Pharmacology, Graduate School of Pharmaceutical Sciences, Kyoto University, Kyoto 606-8501, Japan.
authors.forename: Masakazu
authors.initials: M
authors.lastname: Konno
authors.forename: Hisashi
authors.initials: H
authors.lastname: Shirakawa
authors.forename: Takahito
authors.initials: T
authors.lastname: Miyake
authors.forename: Shinya
authors.initials: S
authors.lastname: Sakimoto
authors.forename: Takayuki
authors.initials: T
authors.lastname: Nakagawa
authors.forename: Shuji
authors.initials: S
authors.lastname: Kaneko
mesh_terms: D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism; D057809:HEK293 Cells; D006801:Humans; D017136:Ion Transport; D008565:Membrane Proteins / Q000378:metabolism*; D009363:Neoplasm Proteins / Q000378:metabolism*; D000071740:ORAI1 Protein; D000071737:Stromal Interaction Molecule 1
chemical_list: D015220:Calcium Channels; D002135:Calcium-Binding Proteins; D008565:Membrane Proteins; D009363:Neoplasm Proteins; D000071740:ORAI1 Protein; C510746:ORAI1 protein, human; C103530:STIM1 protein, human; D000071737:Stromal Interaction Molecule 1; D002118:Calcium