Editors' ChoiceCalcium signaling

Induced Pore Formation by Tetramerization

Sci. Signal.  11 Nov 2008:
Vol. 1, Issue 45, pp. ec382
DOI: 10.1126/scisignal.145ec382

Calcium-release-activated calcium (CRAC) channels are activated in response to depletion of the endoplasmic reticulum calcium stores. The channel is composed of two subunits: the pore-forming Orai subunit and the regulatory Stim subunit. Although it is known that Stim and Orai interact to trigger calcium flux, the structure of Orai is different from that of other channels, and thus the mechanism by which Orai is activated has been unclear. In addition, the stoichiometry of the Orai channel is unclear, with some evidence suggesting a dimer and some suggesting a tetramer. Penna et al. now provide an explanation for these conflicting results. Using biochemical cross-linking methods and single-molecule imaging of fluorescently tagged Orai based on total internal reflection microscopy, the authors demonstrated that in cells expressing only Orai, the protein was predominantly dimeric. When Orai was coexpressed with the C-terminal domain of Stim, which constitutively activates Orai but also prevents the formation of punctae that form when full-length Stim and Orai are coexpressed, Orai was predominantly a tetramer. Although it is possible that Stim triggers the formation of Orai tetramers from independent monomers, the authors favor a model in which Stim triggers the formation of a tetramer from previously formed dimers of Orai. This presents a new mechanism of channel regulation whereby activation and assembly of the functional channel are mediated by the same protein, in this case Stim.

A. Penna, A. Demuro, A. V. Yeromin, S. L. Zhang, O. Safrina, I. Parker, M. D. Cahalan, The CRAC channel consists of a tetramer formed by Stim-induced dimerization of Orai dimers. Nature 456, 116-120 (2008). [PubMed]