The Orai Pore Opening Mechanism

Abstract

Cell survival and normal cell function require a highly coordinated and precise regulation of basal cytosolic Ca2+ concentrations. The primary source of Ca2+ entry into the cell is mediated by the Ca2+ release-activated Ca2+ (CRAC) channel. Its action is stimulated in response to internal Ca2+ store depletion. The fundamental constituents of CRAC channels are the Ca2+ sensor, stromal interaction molecule 1 (STIM1) anchored in the endoplasmic reticulum, and a highly Ca2+-selective pore-forming subunit Orai1 in the plasma membrane. The precise nature of the Orai1 pore opening is currently a topic of intensive research. This review describes how Orai1 gating checkpoints in the middle and cytosolic extended transmembrane regions act together in a concerted manner to ensure an opening-permissive Orai1 channel conformation. In this context, we highlight the effects of the currently known multitude of Orai1 mutations, which led to the identification of a series of gating checkpoints and the determination of their role in diverse steps of the Orai1 activation cascade. The synergistic action of these gating checkpoints maintains an intact pore geometry, settles STIM1 coupling, and governs pore opening. We describe the current knowledge on Orai1 channel gating mechanisms and summarize still open questions of the STIM1–Orai1 machinery.

Isoform-Specific Properties of Orai Homologues in Activation, Downstream Signaling, Physiology and Pathophysiology

“…The pore region is surrounded by a ring of TM2 and TM3 and at the periphery by TM4 regions connected to the helical C-termini [ 61 ] ( Figure 1 E,F). There is profound evidence that pore opening is allosterically affected by checkpoint residues in TM2, TM3 and TM4 [ 12 , 80 , 81 ]. All Orai TM domains include residues, which when mutated can lead to GoF independent of STIM1, clearly indicating that they induce pore opening.…”

“…In line with this, the recent cryo-EM structure [ 68 ] of an Orai open state revealed alterations in the pore region, including a rotation of the hydrophobic cavity and dilation of the basic region. Moreover, LoF mutations, especially at the cytosolic helical extension of the TM regions, including most prominently the cytosolic triangles (see Section 4 ), interfere with pore opening due to reduced hydration of the pore region [ 12 , 81 ]. At the outmost side of the channel complex, the C-terminus is supposed to be connected to TM4 by a bent region [ 61 ] ( Figure 1 C–F), the so-called nexus [ 82 , 83 ], while in the open state, the TM4-C-terminus region is assumed to undergo a conformational change [ 68 , 69 , 70 ].…”

“…Using a library of double mutants, each combining one GoF and one LoF point mutation, we recently showed that the LoF mutation always acted dominantly, thus leading to the abolished activity of the respective double mutants. Hence, we proved that pore opening requires a global conformational change of the entire channel complex [ 12 , 81 , 103 ]. This involves a series of gating checkpoints to adopt an opening permissive conformation.…”

“…This involves a series of gating checkpoints to adopt an opening permissive conformation. A single defect in one of these gating checkpoints independent of their location relative to the pore leads to a loss of function and abolished pore dilation [ 12 , 81 , 103 ]. A comparison of Orai1 and Orai3 revealed that most of the critical gating checkpoints are conserved [ 103 ].…”

“…A prominent Ca 2+ entry pathway into the cell is represented by the Ca 2+ release-activated Ca 2+ (CRAC) ion channel [ 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 ], which is unique due to its composition and structure. It consists of two transmembrane proteins, the Ca 2+ sensor STIM1 located in the ER and the Ca 2+ ion channel Orai1 situated in the plasma membrane.…”