Molecular Modelling of Hexamer and Tetramer Forms of the Orai Calcium Channel

Abstract

In many cell types, calcium influx is carried out by the activation of calcium release-activated calcium (CRAC) channels in the plasma membrane. CRAC channels are assembled from Orai proteins and stromal interaction molecules (STIM). Orai proteins form the transmembrane subunit of CRAC channels, and STIM proteins play a role in their activation. Although recent crystal structure of Orai exhibits a hexameric form with a threefold, quasi-sixfold axial symmetry around the central axis of the channel [Hou et al., Science 338, 1308 (2012)], it has been suggested that only the tetrameric form of Orai achieves calcium selectivity, whereas the hexameric complexes of Orai produce nonselective cationic currents [Thompson and Shuttleworth, Sci. Rep. 3, 1961 (2013)]. In this study, we examine the stability of the hexameric Orai structure and of a model of the tetrameric structure built using the hexamer as a template by performing all-atom molecular dynamics simulations. To explore the functional significance of both forms of the Orai channel (hexamer and tetramer), we analyze the contacts between the channel subunits, the change in pore radius during the simulation, and the occupancy of the pore by water and ions. To investigate ion selectivity, we also calculate the free energy profiles for calcium, potassium and sodium ions permeating through the pore.