Molecular understanding of calcium permeation through the open Orai channel

The Orai channel is characterized by voltage independence, low conductance, and high Ca2+ selectivity and plays an important role in Ca2+ influx through the plasma membrane (PM). How the channel is activated and promotes Ca2+ permeation is not well understood. Here, we report the crystal structure and cryo-electron microscopy (cryo-EM) reconstruction of a Drosophila melanogaster Orai (dOrai) mutant (P288L) channel that is constitutively active according to electrophysiology. The open state of the Orai channel showed a hexameric assembly in which 6 transmembrane 1 (TM1) helices in the center form the ion-conducting pore, and 6 TM4 helices in the periphery form extended long helices. Orai channel activation requires conformational transduction from TM4 to TM1 and eventually causes the basic section of TM1 to twist outward. The wider pore on the cytosolic side aggregates anions to increase the potential gradient across the membrane and thus facilitate Ca2+ permeation. The open-state structure of the Orai channel offers insights into channel assembly, channel activation, and Ca2+ permeation.

Orai通道（Orai channel）具有电压不依赖性、低电导及高Ca²+选择性的特征，在质膜（PM）介导的Ca²+内流过程中发挥重要作用。目前对于该通道的激活机制以及其促进Ca²+渗透的具体过程仍未完全阐明。本研究解析了经电生理学验证为组成型激活的黑腹果蝇Orai（dOrai）突变体（P288L）通道的晶体结构与冷冻电子显微镜（cryo-electron microscopy, cryo-EM）重构结构。Orai通道的开放状态呈六聚体组装形式：中心的6个跨膜螺旋1（TM1）构成离子传导孔道，外周的6个TM4螺旋则形成延伸的长螺旋结构。Orai通道的激活需要跨膜螺旋4（TM4）到跨膜螺旋1（TM1）的构象传递，最终使TM1的碱性区段向外扭转。胞质侧较宽的孔道会聚集阴离子，以提升膜两侧的电位梯度，进而促进Ca²+的渗透。Orai通道的开放态结构为解析通道组装、激活机制以及Ca²+渗透过程提供了重要见解。