Emerging Concepts in Ion Channel Biophysics

Emerging Concepts in Ion Channel Biophysics

Poster Abstracts

109

57-POS

Board 57

Computational Investigation of Hexamer and Tetramer Assemblies of Orai Subunits

Tugba N. Ozturk

1,2

, Guillaume Lamoureux

1,2,3

Concordia University, Montreal, QC, Canada,

Concordia University, Montreal, QC, Canada.

In many cell types, the depletion of intracellular calcium stores triggers a specific form of

calcium influx called store-operated calcium entry, which is crucial for the activation of immune

cells and the function of several cell types. In mast cells, store-operated calcium entry is carried

by calcium release-activated calcium (CRAC) channels. Orai proteins form the pore subunit of

CRAC channels whereas Stromal interaction molecules (STIM) act as sensors for the

intracellular calcium stores.

Although it has been suggested that the functional form of CRAC channels is a tetramer

assembly of Orai subunits, the recent crystal structure of the Orai protein [Hou et al., Science

338, 1308 (2012)] exhibits a hexamer assembly with a threefold, quasi-sixfold axial symmetry

around the ion pore of the channel [Thompson and Shuttleworth, Sci. Rep. 3, 1961 (2013)]. In

addition, the open state of Orai channels is unknown.

In this study, we analyzed the stability and the packing of hexamer and tetramer Orai calcium

channels using all-atom molecular dynamics simulations . We modelled the tetramer assembly of

Orai subunits based on the hexameric crystal structure. Our results indicate that the tetramer Orai

model retains most of the structural features of the hexamer channel, even though the residue-

residue contact analysis reveals a packing less tight than for the Orai hexamer. The tetramer

channel displays a narrower pore containing fewer water molecules, which affects the solvation

of permeant ions. We also computed the free energy profiles of calcium, potassium and sodium

ions permeating through the pore of Orai channel models built with the pore-lining helices in

conformations suggested to be the active form of the channel [Yamashita et al., Nat Commun. 8,

14512 (2017)].