Referenced in: none

Automatically associated channels: Kir2.3 , Slo1

Title: Intramolecular shielding maintains the ER Ca²⁺ sensor STIM1 in an inactive conformation.

Authors: Fang Yu, Lu Sun, Satanay Hubrack, Senthil Selvaraj, Khaled Machaca

Journal, date & volume: J. Cell. Sci., 2013 Jun 1 , 126, 2401-10

PubMed link: http://www.ncbi.nlm.nih.gov/pubmed/23572507

Abstract
Store-operated calcium entry (SOCE) represents a major calcium influx pathway in non-excitable cells and is central to many physiological processes such as T cell activation and mast cell degranulation. SOCE is activated through intricate coordination between the Ca(2+) sensor on the ER membrane (stromal interaction molecule 1, STIM1) and the plasma membrane channel Orai1. When Ca(2+) stores are depleted, STIM1 oligomerizes and physically interacts with Orai1 through its SOAR/CAD domain, resulting in Orai1 gating and Ca(2+) influx. Here, we describe novel inter- and intramolecular FRET sensors in the context of the full-length membrane-anchored STIM1, and show that STIM1 undergoes a conformational change in response to store depletion to adopt a stretched 'open' conformation that exposes SOAR/CAD and allows it to interact with Orai1. Mutational analyses reveal that electrostatic interactions between the predicted first and third coiled-coil domains of STIM1 are not involved in maintaining the 'closed' inactive conformation. In addition, the results argue that an amphipathic α-helix between residues 317 and 336 in the so-called inhibitory domain is important to maintain STIM1 in a closed conformation at rest. Indeed, mutations that alter the amphipathic properties of this helix result in a STIM1 variant that is unable to respond to store depletion in terms of forming puncta, translocation to the cortical ER or activating Orai1.