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Molecular mechanisms of STIM/Orai communication.

Derler I, Jardin I, Romanin C - Am. J. Physiol., Cell Physiol. (2016)

Bottom Line: Functional as well as mutagenesis studies together with structural insights about STIM and Orai proteins provide a molecular picture of the interplay of these two key players in the CRAC signaling cascade.This review focuses on the main experimental advances in the understanding of the STIM1-Orai choreography, thereby establishing a portrait of key mechanistic steps in the CRAC channel signaling cascade.The focus is on the activation of the STIM proteins, the subsequent coupling of STIM1 to Orai1, and the consequent structural rearrangements that gate the Orai channels into the open state to allow Ca(2+)permeation into the cell.

View Article: PubMed Central - PubMed

Affiliation: Institute of Biophysics, Johannes Kepler University of Linz, Linz, Austria; and.

No MeSH data available.


Related in: MedlinePlus

Hypothetical model for gating of Orai1 channels: cartoon representation of the closed (A) and open (B) states of Orai1 when coupled to STIM1-CAD/SOAR or STIM1-CC1α3-CC2. A: in the closed conformation of the Orai1 channel, TM1 is locked in a place that prevents ion permeation. The conformation of TM1 is adjusted by TM2, TM3, and TM4, potentially by the highlighted residues, which can induce constitutive activity upon their mutation. STIM1 assumes the tight, inactive state. B: coupling of Orai1 to STIM1 (here the 2 structurally resolved STIM1 fragments are displayed) induces the open state of Orai1. Both Orai1 NH2-terminal and COOH-terminal interactions with STIM1 are required, to alter the angle at P245 in TM4 and induce TM1 reorientations that switch the hydrophobic gate(s) and render the Orai1 pore conducting. Further alterations in the orientation of the other TM regions are likely to contribute. Focus in this panel is not laid on stoichiometry and how STIM1 couples, but conformational changes occurring upon Orai1 gating are emphasized.
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Figure 4: Hypothetical model for gating of Orai1 channels: cartoon representation of the closed (A) and open (B) states of Orai1 when coupled to STIM1-CAD/SOAR or STIM1-CC1α3-CC2. A: in the closed conformation of the Orai1 channel, TM1 is locked in a place that prevents ion permeation. The conformation of TM1 is adjusted by TM2, TM3, and TM4, potentially by the highlighted residues, which can induce constitutive activity upon their mutation. STIM1 assumes the tight, inactive state. B: coupling of Orai1 to STIM1 (here the 2 structurally resolved STIM1 fragments are displayed) induces the open state of Orai1. Both Orai1 NH2-terminal and COOH-terminal interactions with STIM1 are required, to alter the angle at P245 in TM4 and induce TM1 reorientations that switch the hydrophobic gate(s) and render the Orai1 pore conducting. Further alterations in the orientation of the other TM regions are likely to contribute. Focus in this panel is not laid on stoichiometry and how STIM1 couples, but conformational changes occurring upon Orai1 gating are emphasized.

Mentions: The external vestibule (see Fig. 2C) is formed by the TM1-TM2 loops, each of which includes three negatively charged residues (D110, D112, D114). Their triple mutation to alanines alters ion selectivity and induces a widening of the pore (179), while single cysteine or alanine substitutions leave the Orai1 mutants' Ca2+ selectivity unaltered (51, 99). These loop segments have been shown via the substituted cysteine accessibility method (SCAM) to tightly couple with large (>8 Å) as well as small (<3 Å) positively as well as negatively charged MTS reagents (99). These results suggest that the first loops surround a large vestibule that is able to accommodate bulky compounds of different size and charge. Furthermore, disulfide cross-linking studies as well as Cd+ block experiments have suggested that these loop regions are very flexible (102). Recent data by Frischauf et al. (51) have revealed that these negatively charged residues (D110, D112, D114) function as a Ca2+ accumulating region (CAR) of Orai channels (Fig. 4), probably decreasing the energetic barrier for Ca2+ ions to enter the pore. In particular, D110 and D112 have been demonstrated by MD simulations to function as transient Ca2+ binding sites before Ca2+ ions reach the selectivity filter located 1.2 nm away from CAR. A substitution of D110 to an alanine results in reduced permeation of Ca2+, particularly at lower extracellular Ca2+ concentrations, likely because of a shift of Ca2+ binding to D112 and D114 and thus an enhanced energy barrier between CAR and the selectivity filter. Besides the interaction of the TM1-TM2 loop1 with Ca2+, additional electrostatic interactions have been discovered between TM1-TM2 loop1 and the TM3-TM4 connecting loop3, enabling fine-tuning of Ca2+ accumulation to the pore. Cysteine-induced cross-linking of D112 and R210 results in reduced store-operated current densities, which were enhanced by 400% upon breakage of disulfide bonds. Hence, the loop3 apparently competes with Ca2+ binding to loop1, thereby finely adjusting the Ca2+ permeation (51).


Molecular mechanisms of STIM/Orai communication.

Derler I, Jardin I, Romanin C - Am. J. Physiol., Cell Physiol. (2016)

Hypothetical model for gating of Orai1 channels: cartoon representation of the closed (A) and open (B) states of Orai1 when coupled to STIM1-CAD/SOAR or STIM1-CC1α3-CC2. A: in the closed conformation of the Orai1 channel, TM1 is locked in a place that prevents ion permeation. The conformation of TM1 is adjusted by TM2, TM3, and TM4, potentially by the highlighted residues, which can induce constitutive activity upon their mutation. STIM1 assumes the tight, inactive state. B: coupling of Orai1 to STIM1 (here the 2 structurally resolved STIM1 fragments are displayed) induces the open state of Orai1. Both Orai1 NH2-terminal and COOH-terminal interactions with STIM1 are required, to alter the angle at P245 in TM4 and induce TM1 reorientations that switch the hydrophobic gate(s) and render the Orai1 pore conducting. Further alterations in the orientation of the other TM regions are likely to contribute. Focus in this panel is not laid on stoichiometry and how STIM1 couples, but conformational changes occurring upon Orai1 gating are emphasized.
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Related In: Results  -  Collection

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Show All Figures
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Figure 4: Hypothetical model for gating of Orai1 channels: cartoon representation of the closed (A) and open (B) states of Orai1 when coupled to STIM1-CAD/SOAR or STIM1-CC1α3-CC2. A: in the closed conformation of the Orai1 channel, TM1 is locked in a place that prevents ion permeation. The conformation of TM1 is adjusted by TM2, TM3, and TM4, potentially by the highlighted residues, which can induce constitutive activity upon their mutation. STIM1 assumes the tight, inactive state. B: coupling of Orai1 to STIM1 (here the 2 structurally resolved STIM1 fragments are displayed) induces the open state of Orai1. Both Orai1 NH2-terminal and COOH-terminal interactions with STIM1 are required, to alter the angle at P245 in TM4 and induce TM1 reorientations that switch the hydrophobic gate(s) and render the Orai1 pore conducting. Further alterations in the orientation of the other TM regions are likely to contribute. Focus in this panel is not laid on stoichiometry and how STIM1 couples, but conformational changes occurring upon Orai1 gating are emphasized.
Mentions: The external vestibule (see Fig. 2C) is formed by the TM1-TM2 loops, each of which includes three negatively charged residues (D110, D112, D114). Their triple mutation to alanines alters ion selectivity and induces a widening of the pore (179), while single cysteine or alanine substitutions leave the Orai1 mutants' Ca2+ selectivity unaltered (51, 99). These loop segments have been shown via the substituted cysteine accessibility method (SCAM) to tightly couple with large (>8 Å) as well as small (<3 Å) positively as well as negatively charged MTS reagents (99). These results suggest that the first loops surround a large vestibule that is able to accommodate bulky compounds of different size and charge. Furthermore, disulfide cross-linking studies as well as Cd+ block experiments have suggested that these loop regions are very flexible (102). Recent data by Frischauf et al. (51) have revealed that these negatively charged residues (D110, D112, D114) function as a Ca2+ accumulating region (CAR) of Orai channels (Fig. 4), probably decreasing the energetic barrier for Ca2+ ions to enter the pore. In particular, D110 and D112 have been demonstrated by MD simulations to function as transient Ca2+ binding sites before Ca2+ ions reach the selectivity filter located 1.2 nm away from CAR. A substitution of D110 to an alanine results in reduced permeation of Ca2+, particularly at lower extracellular Ca2+ concentrations, likely because of a shift of Ca2+ binding to D112 and D114 and thus an enhanced energy barrier between CAR and the selectivity filter. Besides the interaction of the TM1-TM2 loop1 with Ca2+, additional electrostatic interactions have been discovered between TM1-TM2 loop1 and the TM3-TM4 connecting loop3, enabling fine-tuning of Ca2+ accumulation to the pore. Cysteine-induced cross-linking of D112 and R210 results in reduced store-operated current densities, which were enhanced by 400% upon breakage of disulfide bonds. Hence, the loop3 apparently competes with Ca2+ binding to loop1, thereby finely adjusting the Ca2+ permeation (51).

Bottom Line: Functional as well as mutagenesis studies together with structural insights about STIM and Orai proteins provide a molecular picture of the interplay of these two key players in the CRAC signaling cascade.This review focuses on the main experimental advances in the understanding of the STIM1-Orai choreography, thereby establishing a portrait of key mechanistic steps in the CRAC channel signaling cascade.The focus is on the activation of the STIM proteins, the subsequent coupling of STIM1 to Orai1, and the consequent structural rearrangements that gate the Orai channels into the open state to allow Ca(2+)permeation into the cell.

View Article: PubMed Central - PubMed

Affiliation: Institute of Biophysics, Johannes Kepler University of Linz, Linz, Austria; and.

No MeSH data available.


Related in: MedlinePlus