Store-operated calcium channels: new perspectives on mechanism and function

Abstract

Store-operated calcium channels (SOCs) are a nearly ubiquitous Ca(2+) entry pathway stimulated by numerous cell surface receptors via the reduction of Ca(2+) concentration in the ER. The discovery of STIM proteins as ER Ca(2+) sensors and Orai proteins as structural components of the Ca(2+) release-activated Ca(2+) (CRAC) channel, a prototypic SOC, opened the floodgates for exploring the molecular mechanism of this pathway and its functions. This review focuses on recent advances made possible by the use of STIM and Orai as molecular tools. I will describe our current understanding of the store-operated Ca(2+) entry mechanism and its emerging roles in physiology and disease, areas of uncertainty in which further progress is needed, and recent findings that are opening new directions for research in this rapidly growing field.

Figure 1.

The molecular choreography of store-operated calcium entry. (A) In this resting HEK 293 cell, mCherry-STIM1 is localized throughout the ER and eGFP-Orai1 is dispersed throughout the PM of the cell footprint. Following store depletion, both proteins redistribute into colocalized puncta. (Panel A is from Park et al. (2009) and reprinted with permission from Elsevier © 2009.) (B) Electron micrograph showing accumulation of HRP-STIM1 in the ER (arrows) at ER-PM junctions in a Jurkat T cell after store depletion. (Panel B is modified from Luik et al. (2006) and reprinted with permission from The Rockefeller University Press © 2006.) (C) A current model for SOCE, divided into four phases from left to right. For simplicity and to emphasize the interactions of STIM1 and Orai1, full stoichiometries are not shown (STIM1 is likely to be a dimer at rest and after store depletion at least a tetramer, and 8 STIM1s probably interact with each CRAC channel for full activation). At far left, STIM1 is pictured in its resting state when Ca²⁺ stores are replete. The Ca²⁺-bound EF hand interacts with the SAM domain, and electrostatic interactions between acidic residues in CC1 (−) and basic residues in CAD (+) prevent CAD from interacting with Orai. On store depletion, Ca²⁺ is released from the EF hand, allowing STIM1 to oligomerize (shown here schematically as a dimer) and assume an extended conformation that exposes CAD and the polybasic domain. Oligomers move to ER-PM junctions by diffusion in the ER membrane, and accumulate there through interaction of the polybasic domain with phosphoinositides in the PM. At the junction, CRAC channels diffusing in the PM bind to the STIM1 CAD via electrostatic interaction of a coiled-coil region of the Orai1 carboxyl terminus (−) with basic CAD residues (+). This interaction traps CRAC channels and combined with CAD interactions with the Orai1 amino terminus (not shown) opens them at the ER-PM junction. EF, canonical EF hand; SAM, sterile α-motif; CC1, coiled-coil 1; CAD, CRAC activation domain; PBD, polybasic domain.

Figure 2.

Functional organization of STIM1. The major functional domains of STIM1 are indicated by residue numbers relative to the translation initiation site. SP, signal peptide; cEF1, canonical EF hand; hEF2, hidden (noncanonical) EF hand; SAM, sterile α-motif; TM, transmembrane domain; CC1–3, coiled-coil domains 1–3; CAD, CRAC activation domain; ID, inactivation domain; P/S, proline-serine-rich domain; K, polybasic domain. CAD is highly similar to SOAR (aa 344–442) and Ccb9 (aa 339–444), not shown. Sequences are displayed for regions with established functions: the Ca²⁺ sensing domain in cEF1, the basic region of CC2 involved in autoinhibitory binding to the acidic region of CC1 and stimulatory binding to Orai1, the acidic residues in the ID of STIM1 required for Ca²⁺-dependent inactivation, the EB1 binding sequence (TRIP) that links STIM1 to the tips of growing microtubules, and the carboxy-terminal polybasic domain that targets STIM1 to the ER-PM junction. Phosphorylation sites S486 and S668 help suppress STIM1 activity and SOCE during mitosis. Acidic and basic residues are shown in red and blue, respectively.

Figure 3.

Functional organization of Orai1. This schematic layout of a single Orai1 subunit includes the four transmembrane domains (TM1–TM4), amino and carboxyl termini, and the connecting loops. Functionally significant residues and sequences are shown. The indicated residues in TM1 line the aqueous pore, E106 (red) forms the ion selectivity filter, and R91 inhibits channel function when substituted by bulky hydrophobic residues. D110/112/114 help determine sensitivity to block by lanthanides. In the amino terminus proximal to the PM, residues 68–91 form a Ca²⁺/CaM binding domain, with residues essential for CaM binding and CDI indicated in bold green (A73, W76, Y80). This region also binds CAD and contains residues required for channel opening in bold blue (K85, R91). A domain in the intracellular loop is required for CDI, and a probable coiled-coil domain in the carboxyl terminus contains hydrophobic (bold green) and acidic (bold red) residues critical for binding STIM1 CAD.