Highlighting the Multifaceted Role of Orai1 N-Terminal- and Loop Regions for Proper CRAC Channel Functions

Abstract

Orai1, the Ca²⁺-selective pore in the plasma membrane, is one of the key components of the Ca²⁺release-activated Ca²⁺ (CRAC) channel complex. Activated by the Ca²⁺ sensor in the endoplasmic reticulum (ER) membrane, stromal interaction molecule 1 (STIM1), via direct interaction when ER luminal Ca²⁺ levels recede, Orai1 helps to maintain Ca²⁺ homeostasis within a cell. It has already been proven that the C-terminus of Orai1 is indispensable for channel activation. However, there is strong evidence that for CRAC channels to function properly and maintain all typical hallmarks, such as selectivity and reversal potential, additional parts of Orai1 are needed. In this review, we focus on these sites apart from the C-terminus; namely, the second loop and N-terminus of Orai1 and on their multifaceted role in the functioning of CRAC channels.

Keywords: CRAC channel; Loop2; N-terminus; Orai1; calcium.

Figure 1

Schematic on the topology of Orai1 channels, highlighting the positions of functionally important residues. For the sake of simplicity, only two opposing Orai1 proteins are shown. The acidic residues D110/D112/D114 reside at the extracellular mouth of the pore and serve as Ca²⁺ accumulating region (CAR), while E106 defines the narrowest area along the pore and functions as a selectivity filter. Further in direction towards the cytosol, the pore is lined by the hydrophobic residues V102, F99 and L95, as well as the subsequent basic side chains of R91, K87 and R83, respectively. N223 in the third loop represents a glycosylation site, while the cysteines at the positions 126, 143 and 195, are involved in redox regulation of Orai1. P245 disrupts the continuity of the fourth transmembrane helix and may lead to constitutive activity when mutated. The five subsequent residues ₂₆₁LVSHK₂₆₅ define the nexus domain, which is relevant for gating as well (see text for further details).

Figure 2

Overview on Orai1 N-terminal or loop2 regions involved in interactions with internal proteins, gating, the regulation by lipids or intracellular pH and, among others, CRAC channel inactivation. AC8: adenylyl cyclase 8, AKAP: A-kinase anchoring protein, Cav: Caveolin, CaM: Calmodulin, CCT: chaperonin-containing T-complex protein 1, CRACR2A: CRAC regulator 2A, ETON: extended transmembrane Orai1 N-terminal region, FCDI: fast Ca²⁺ dependent inactivation, SOGI: STIM1-Orai1 gating interface, TM: transmembrane.