Ion Permeation through ORAI Proteins

Abstract

Under depleted calcium levels in the endoplasmic reticulum, calcium release-activated calcium (CRAC) channels generate a sustained calcium influx crucial for several cellular responses such as gene expression, cell migration, and growth. Abnormal CRAC channel activity has been linked to various diseases, including severe combined immunodeficiency and autoimmunity disorders, allergy, and inflammatory bowel disease. CRAC channels are composed of two proteins: Orai proteins, that form the pore subunit, and Stim proteins, that activate the pore. Recent crystal structures of Orai proteins have shed light onto the nature of the permeation pathway of CRAC channels but details of the ion permeation mechanism are yet to be revealed. In this study, we use Umbrella Sampling and the Weighted Histogram Analysis Method to investigate the permeation of calcium, potassium and sodium ions through Orai pores. We extend the calculations to both Orai hexamer and tetramer assemblies, since previous studies have implicated different multimeric states of Orai as functional CRAC channels. In addition, we investigate the effect of rotation of the pore-forming transmembrane helices (suggested to be responsible for gating) on the free energy cost of ion permeation. Our simulations indicate that both hexamer and tetramer assemblies form conducting ion pores in which the free energy barrier is mainly set by the hydrophobic core of the pore. In all cases, helix rotation lowers the energetic cost of translocating an ion.