Ca(2+) release-activated Ca(2+) (CRAC) current, structure, and function

Abstract

Store-operated Ca(2+) entry describes the phenomenon that connects a depletion of internal Ca(2+) stores to an activation of plasma membrane-located Ca(2+) selective ion channels. Tremendous progress towards the underlying molecular mechanism came with the discovery of the two respective limiting components, STIM and Orai. STIM1 represents the ER-located Ca(2+) sensor and transmits the signal of store depletion to the plasma membrane. Here it couples to and activates Orai, the highly Ca(2+)-selective pore-forming subunit of Ca(2+) release-activated Ca(2+) channels. In this review, we focus on the molecular steps that these two proteins undergo from store-depletion to their coupling, the activation, and regulation of Ca(2+) currents.

Fig. 1

Predicted functional domains within human STIM1. From left to right: EF canonical/hidden EF-hand motif, SAM sterile alpha motif, TM transmembrane domain, CC1/CC2/CC3 coiled-coil domains 1–3, CMD CRAC modulatory domain, ID _STIM inactivation domain of STIM1, SHD STIM1 homomerization domain, S/P serine/proline-rich region, K polybasic cluster. The minimal functional regions within STIM1 are highlighted on the top: CAD CRAC activating domain, SOAR stim Orai-activating region, Ccb9 coiled-coil domain region containing region b9, OASF Orai-activating small fragment. The respective sequences include amino acids that have been reported to play a crucial role in STIM1 activation and function (charged amino acids are highlighted in blue (−) or red (+))

Fig. 2

Predicted functional domains within Orai1. Schematic depiction of a single Orai1 subunit with selectively highlighted residues that are critical for channel function (blue (−), red (+), yellow (hydrophobic)). In detail, the conserved region (thick purple line) on the distal end of the amino terminus contains crucial amino acids for channel gating (K85, R91) as well as Ca²⁺-dependent CaM binding (A73, W76, Y80). Transmembrane domain 1 (TM1) buries the pore-forming residues and the selectivity filter whereas transmembrane domain 3 (TM3) might allosterically affect channel gating. The carboxyl terminus comprises charged (E272, E275, E278) as well as hydrophobic (L273, L276) amino acids that possibly account for coiled-coil domain formation and STIM coupling

Fig. 3

Model for STIM1 activation and coupling to Orai1. This model emphasizes domain interactions mediating the coupling between STIM1 and Orai rather than depicting the full stoichiometry of the complete tetrameric Orai1 assembly. On the left, under resting state conditions in which Ca²⁺-stores are replete, STIM1 is considered to be in a closed conformation. Intramolecular locking involves hydrogen bonds between an inhibitory helix (blue) at the C-terminal end of the first coiled-coil with the CAD/SOAR region. The cluster of positive residues (+/red) and hydrophobic (yellow) interactions via the second and third extended coiled-coil domains arrange the CAD/SOAR dimer in a V-shape form. On the right, Ca²⁺-store-depletion triggers STIM1 oligomerization by refolding of EF-SAM domain followed by a transition of the cytosolic portion into an extended conformation. This causes the rearrangement of STIM1 into punctate clusters and attraction of Orai1 to these regions via a direct interaction of their carboxyl termini. A second additional binding to the N-terminus of Orai1 culminates in channel activation and Ca²⁺ influx