Mapping interactions between the CRAC activation domain and CC1 regulating the activity of the ER Ca2+ sensor STIM1

Stromal interaction molecule 1 (STIM1) is a widely expressed protein that functions as the endoplasmic reticulum (ER) Ca2+ sensor and activator of Orai1 channels. In resting cells with replete Ca2+ stores, an inhibitory clamp formed by the coiled-coil 1 (CC1) domain interacting with the CRAC-activation domain (CAD) of STIM1 helps keep STIM1 in a quiescent state. Following depletion of ER Ca2+ stores, the brake is released, allowing CAD to extend away from the ER membrane and enabling it to activate Orai1 channels. However, the molecular determinants of CC1-CAD interactions that enforce the inhibitory clamp are incompletely understood. Here, we performed Ala mutagenesis in conjunction with live-cell FRET analysis to examine residues in CC1 and CAD that regulate the inhibitory clamp. Our results indicate that in addition to previously identified hotspots in CC1⍺1 and CC3, several hydrophobic residues in CC2 and the apex region of CAD are critical for CC1-CAD interactions. Mutations in these residues loosen the CC1-CAD inhibitory clamp to release CAD from CC1 in cells with replete Ca2+ stores. By contrast, altering the hydrophobic residues L265 and L273 strengthens the clamp to prevent STIM1 activation. Inclusion of the inactivation domain of STIM1 helps stabilize CC1-CAD interaction in several mutants to prevent spontaneous STIM1 activation. In addition, R426C, a human disease-linked mutation in CC3, affects the clamp but also impairs Orai1 binding to inhibit CRAC channel activation. These results identify the CC2, apex, and inactivation domain regions of STIM1 as important determinants of STIM1 activation.

An open pore structure of the Orai channel, finally

Store-operated Orai channels are a primary mechanism for mobilizing Ca²⁺ signals in both non-excitable cells and excitable cells. The structure of the open channel, vital for understanding the mechanism of channel opening, is incompletely understood. We highlight a new study that unveils the structure of a constitutively active Orai mutant and takes us closer towards understanding the molecular basis of Orai channel activation.

The basic residues in the Orai1 channel inner pore promote opening of the outer hydrophobic gate

CRAC channels contain a cluster of positively charged residues in the inner pore whose function is not understood. Here, we show that these positive charges promote pore opening by enhancing hydration of the hydrophobic gate located at the outer end of the pore.

Store-operated Orai1 channels regulate a wide range of cellular functions from gene expression to cell proliferation. Previous studies have shown that gating of Orai1 channels is regulated by the outer pore residues V102 and F99, which together function as a hydrophobic gate to block ion conduction in resting channels. Opening of this gate occurs through a conformational change that moves F99 away from the permeation pathway, leading to pore hydration and ion conduction. In addition to this outer hydrophobic gate, several studies have postulated the presence of an inner gate formed by the basic residues R91, K87, and R83 in the inner pore. These positively charged residues were suggested to block ion conduction in closed channels via mechanisms involving either electrostatic repulsion or steric occlusion by a bound anion plug. However, in contrast to this model, here we find that neutralization of the basic residues dose-dependently abolishes both STIM1-mediated and STIM1-independent activation of Orai1 channels. Molecular dynamics simulations show that loss of the basic residues dehydrates the pore around the hydrophobic gate and stabilizes the pore in a closed configuration. Likewise, the severe combined immunodeficiency mutation, Orai1 R91W, closes the channel by dewetting the hydrophobic stretch of the pore and stabilizing F99 in a pore-facing configuration. Loss of STIM1-gating in R91W and in the other basic residue mutants is rescued by a V102A mutation, which restores pore hydration at the hydrophobic gate to repermit ion conduction. These results indicate that the inner pore basic residues facilitate opening of the principal outer hydrophobic gate through a long-range effect involving hydration of the outer pore.

MCU meets cardiolipin: Calcium and disease follow form

Cardiolipin (CL) is a cone-shaped lipid found nearly exclusively in the inner mitochondrial membrane of animal cells. Disruption of CL synthesis leads to abnormalities in mitochondrial shape and function, but the underlying causes are incompletely understood. We highlight a new study that reveals that the activity of the mitochondrial calcium uniporter (MCU) is regulated by CL, advancing our understanding of the mechanisms of CL-linked human disease.

Molecular basis of allosteric Orai1 channel activation by STIM1

Store-operated Ca²⁺ entry through Orai1 channels is a primary mechanism for Ca²⁺ entry in many cells and mediates numerous cellular effector functions ranging from gene transcription to exocytosis. Orai1 channels are amongst the most Ca²⁺ -selective channels known and are activated by direct physical interactions with the endoplasmic reticulum Ca²⁺ sensor stromal interaction molecule 1 (STIM1) in response to store depletion triggered by stimulation of a variety of cell surface G-protein coupled and tyrosine kinase receptors. Work in the last decade has revealed that the Orai1 gating process is highly cooperative and strongly allosteric, likely driven by a wave of interdependent conformational changes throughout the protein originating in the peripheral C-terminal ligand binding site and culminating in pore opening. In this review, we survey the structural and molecular features in Orai1 that contribute to channel gating and consider how they give rise to the unique biophysical fingerprint of Orai1 currents.

The exquisitely cooperative nature of Orai1 channel activation

Yeung and Prakriya highlight new research showing that STIM1 must bind to all six Orai1 subunits to effectively activate the channel.