Crystal Structure of the Calcium Release–Activated Calcium Channel Orai

Abstract

The plasma membrane protein Orai forms the pore of the CRAC channel (calcium release-activated calcium channel) and generates sustained cytosolic calcium signals when triggered by depletion of calcium from the endoplasmic reticulum. The crystal structure of Orai from Drosophila melanogaster, determined at 3.35 angstrom resolution, reveals that the calcium channel is comprised of a hexameric assembly of Orai subunits arranged around a central ion pore. The pore traverses the membrane and extends 20 angstroms into the cytosol. A ring of glutamates on its extracellular side forms the selectivity filter. A basic region near the intracellular side can bind anions that may stabilize the closed state. The architecture of the channel differs markedly from other ion channels and gives insight into the principles of selective calcium permeation and gating.

Orai1 pore residues control CRAC channel inactivation independently of calmodulin

“…Shortly after publication of the Ca 2+ -CaM-CBD Orai1 structure, the crystal structure of the full Drosophila melanogaster Orai1 (dOrai) channel revealed a hexameric subunit stoichiometry and a very long pore made of TM1 helices predicted to extend >20 Å beyond the membrane into the cell cytoplasm (Hou et al, 2012). Surprisingly, the side chains of residues corresponding to human W76 and Y80 were shown to face the pore lumen rather than being freely accessible to cytosolic CaM as previously assumed.…”

“…However, we can estimate i Ca by multiplying i Na by the ratio of Ca 2+ and Na + macroscopic currents, with a small correction Table 1. Charge reversal or neutralization in the triple basic ring essentially eliminates CDI A distinctive feature of the hexameric dOrai structure is a triple ring of positively charged side chains lining the central pore (R155, K159, and K163, corresponding to human Orai1 R83, K87, and R91, respectively; Hou et al, 2012). This triple ring is immediately adjacent to residues W76 and Y80.…”

“…Fig. 1 A shows a representation of the hexameric dOrai crystal structure (Hou et al, 2012) viewed from the cytosol, with an inner ring of six transmembrane TM1 domains that line the pore, surrounded by TM2, TM3, and TM4 domains; the aligned human CBD Orai derived from the Ca 2+ -CaM-CBD Orai1 crystal structure (Liu et al, 2012) is shown in yellow. Fig.…”

“…The relationship between the Ca 2+ -CaM-CBD Orai1 crystal structure (Liu et al, 2012) and the full-length hexameric dOrai crystal structure (Hou et al, 2012) is important for evaluating possible mechanisms of CRAC channel CDI. We aligned the two structures using the CBD regions of hOrai1 and dOrai to explore the plausibility of Ca 2+ -CaM binding to the hexameric Orai1 pore.…”

“…2 A, top; P < 0.01 at 100, Structural modeling In Fig. 1, the human CaM/CBD Orai1 structure 4EHQ (Liu et al, 2012) was aligned with the dOrai structure 4HKR (Hou et al, 2012) using the ALIGN function of MacPyMOL v1.5.0.5 and coordinates from the Protein Data Bank.…”