Identifying the CRAC Channel

Abstract

Release of Ca 2+ from intracellular stores elicits Ca 2+ entry through plasma membrane Ca 2+ release-activated Ca 2+ (CRAC) channels. Stromal interaction molecule (Stim in Drosophila , STIM1 in mammals) and Orai (in Drosophila , Orai1 in humans, also known as CRACM1) have been implicated in mediating the CRAC current; however, the precise relation of Orai to the CRAC channel has been unclear. Two groups now use mutational analysis to identify Orai as forming the CRAC channel pore subunit. Yeromin et al . overexpressed wild-type or mutant Orai together with Stim in S1 cells. Store depletion promoted association of the wild-type proteins and elicited a large CRAC current; coexpression of Stim with Orai mutants altered CRAC current properties. A glutamate to aspartate mutation at position 180 (E180D) altered ion selectivity and rectification; a glutamate to alanine mutation at the same location acted as a dominant negative to block native CRAC current; and mutations of aspartate to alanine at 184 and 186 or asparagine to alanine at 188 reduced sensitivity to Gd 3+ block. Prakirya et al . used both biotinylation with a cell-impermeant reagent and immunocytochemical staining of tagged Orai1 expressed in human embryonic kidney (HEK) 293 cells to show that it was a plasma membrane protein. They found that a glutamate to alanine mutation at position 106 (E106D, corresponding to Yeromin et al .’s E180D mutation) altered ion selectivity of Orai1 expressed in T cells deficient in CRAC current and Ca 2+ -influx, as did a glutamate to glutamine mutation at position 190. Thus, both groups conclude that Orai forms the CRAC channel pore subunit. M. Prakriya, S. Feske, Y. Gwack, S. Srikanth, A. Rao, P. G. Hogan, Orai1 is an essential pore subunit of the CRAC channel. Nature 443 , 230-233 (2006). [PubMed] A. V. Yeromin, S. L. Zhang, W. Jiang, Y. Yu, O. Safrina, M. D. Cahalan, Molecular identification of the CRAC channel by altered ion selectivity in a mutant of Orai. Nature 443 , 226-229 (2006). [PubMed]