Inherited disease-linked arginine76/75 mutants in Cx50 and Cx45 showed impaired homotypic and heterotypic gap junction function, but not Cx43

Connexins form intercellular communication channels, known as gap junctions (GJs), in many tissues/organs. Mutations in connexin genes are found to be linked to various inherited diseases, but the mechanisms are not fully clear. The Arg76 (R76) in Cx50 is fully conserved across the entire connexin family and is a hotspot for five connexin-linked inherited diseases, including Cx50 and Cx46-linked congenital cataract, Cx43-linked oculodentodigital dysplasia, and Cx45-linked cardiac arrhythmias. To better understand the molecular and cellular mechanism of dysfunction caused by R76/75 mutations, we examined the functional status and properties of GJs containing R76 mutations in Cx50 (R76H/C), Cx43 (R76H/S/C), and Cx45 (R75H) with an emphasis on heterotypic GJs in connexin-deficient model cells. All tested mutants showed an impairment of homotypic GJ function reflected by a decreased coupling% and conductance, except for Cx43 R76H/S. These connexin mutants also showed impaired GJ function when paired with a docking compatible connexin, such as Cx50/Cx46 or Cx45/Cx43, except for all mutants on Cx43 which formed functional heterotypic GJs with Cx45. Localization studies on fluorescent protein tagged connexin mutants revealed that Cx45 R75H and Cx43 R76C showed impaired localization. Our homology structure models indicated that mutations of R76/75 in these GJs led to a loss of intra- and/or inter-connexin non-covalent interactions (salt bridges) at the sidechain of this residue, which could contribute to the observed GJ impairments underlying diseases. It is interesting that unlike those disease-linked variants in Cx50 and Cx45, Cx43 can tolerate some variations at R76.

A dual mechanism promotes switching of the Stormorken STIM1 R304W mutant into the activated state

STIM1 and Orai1 are key components of the Ca²⁺-release activated Ca²⁺ (CRAC) current. Orai1, which represents the subunit forming the CRAC channel complex, is activated by the ER resident Ca²⁺ sensor STIM1. The genetically inherited Stormorken syndrome disease has been associated with the STIM1 single point R304W mutant. The resulting constitutive activation of Orai1 mainly involves the CRAC-activating domain CAD/SOAR of STIM1, the exposure of which is regulated by the molecular interplay between three cytosolic STIM1 coiled-coil (CC) domains. Here we present a dual mechanism by which STIM1 R304W attains the pathophysiological, constitutive activity eliciting the Stormorken syndrome. The R304W mutation induces a helical elongation within the CC1 domain, which together with an increased CC1 homomerization, destabilize the resting state of STIM1. This culminates, even in the absence of store depletion, in structural extension and CAD/SOAR exposure of STIM1 R304W leading to constitutive CRAC channel activation and Stormorken disease.

Stormorken syndrome is associated with the R304W mutation in STIM1, which is a Calcium sensor in the endoplasmic reticulum. Here authors use FRET and electrophysiology to show that R304W induces STIM1 conformational extension by a dual mechanism resulting in constitutive activation of Ca2+ channels.

A coiled-coil clamp controls both conformation and clustering of stromal interaction molecule 1 (STIM1)

Store-operated Ca²⁺ entry, essential for the adaptive immunity, is initiated by the endoplasmic reticulum (ER) Ca²⁺ sensor STIM1. Ca²⁺ entry occurs through the plasma membrane resident Ca²⁺ channel Orai1 that directly interacts with the C-terminal STIM1 domain, named SOAR/CAD. Depletion of the ER Ca²⁺ store controls this STIM1/Orai1 interaction via transition to an extended STIM1 C-terminal conformation, exposure of the SOAR/CAD domain, and STIM1/Orai1 co-clustering. Here we developed a novel approach termed FRET-derived Interaction in a Restricted Environment (FIRE) in an attempt to dissect the interplay of coiled-coil (CC) interactions in controlling STIM1 quiescent as well as active conformation and cluster formation. We present evidence of a sequential activation mechanism in the STIM1 cytosolic domains where the interaction between CC1 and CC3 segment regulates both SOAR/CAD exposure and CC3-mediated higher-order oligomerization as well as cluster formation. These dual levels of STIM1 auto-inhibition provide efficient control over the coupling to and activation of Orai1 channels.

STIM1 couples to ORAI1 via an intramolecular transition into an extended conformation

Upon depletion of ER calcium stores, STIM1 and ORAI1 associate and induce calcium release-activated calcium (CRAC) currents. This study reveals that STIM1 undergoes an intramolecular transition into an extended conformation that is involved in ORAI1 binding and activation.

Stromal interaction molecule (STIM1) and ORAI1 are key components of the Ca²⁺ release-activated Ca²⁺ (CRAC) current having an important role in T-cell activation and mast cell degranulation. CRAC channel activation occurs via physical interaction of ORAI1 with STIM1 when endoplasmic reticulum Ca²⁺ stores are depleted. Here we show, utilizing a novel STIM1-derived Förster resonance energy transfer sensor, that the ORAI1 activating small fragment (OASF) undergoes a C-terminal, intramolecular transition into an extended conformation when activating ORAI1. The C-terminal rearrangement of STIM1 does not require a functional CRAC channel, suggesting interaction with ORAI1 as sufficient for this conformational switch. Extended conformations were also engineered by mutations within the first and third coiled-coil domains in the cytosolic portion of STIM1 revealing the involvement of hydrophobic residues in the intramolecular transition. Corresponding full-length STIM1 mutants exhibited enhanced interaction with ORAI1 inducing constitutive CRAC currents, even in the absence of store depletion. We suggest that these mutant STIM1 proteins imitate a physiological activated state, which mimics the intramolecular transition that occurs in native STIM1 upon store depletion.