A Molecular Mechanism for Orai1 Channel Activation by STIM1

Abstract

The calcium release activated calcium (CRAC) channel plays a key role in diverse cellular functions. Assembly of endoplasmic reticulum-plasma membrane (ER-PM) bridges between ER-resident STIM1 proteins and PM-resident Orai1 proteins translate calcium depletion in ER into Orai1 channel opening and replenishment of cellular calcium. Gating of Orai1 by STIM1 occurs in a graded and non-linear manner which is poorly understood. Using electrophysiological, optical and biochemical techniques, we examined the functional roles of regions of the STIM1-Orai1 activating region (SOAR) of STIM1. We find that SOAR mutants that are deficient in binding and activation of resting Orai1 channels are able to bind to and boost activation of partially activated Orai1 channels. We further show that the STIM1 binding regions on Orai1 undergo structural rearrangement during channel activation. The results suggest that activation of Orai1 by SOAR occurs in multiple steps. A first step in which SOAR binding to Orai1 partially activates the channel and induces a rearrangement in the SOAR binding site of Orai1. That rearrangement of Orai1 then permits sequential steps of SOAR binding, via distinct molecular interactions, to fully activate the channel.