title: Intramolecular shielding maintains the ER Ca²⁺ sensor STIM1 in an inactive conformation.
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.
authors.affiliation: Department of Physiology and Biophysics, Weill Cornell Medical College in Qatar, Qatar Foundation, Education City, Doha, Qatar.
authors.forename: Fang
authors.initials: F
authors.lastname: Yu
authors.forename: Lu
authors.initials: L
authors.lastname: Sun
authors.forename: Satanay
authors.initials: S
authors.lastname: Hubrack
authors.forename: Senthil
authors.initials: S
authors.lastname: Selvaraj
authors.forename: Khaled
authors.initials: K
authors.lastname: Machaca
mesh_terms: D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D057809:HEK293 Cells; D006801:Humans; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D009154:Mutation; D009363:Neoplasm Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D000071740:ORAI1 Protein; D055503:Protein Multimerization / Q000502:physiology*; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D021381:Protein Transport / Q000502:physiology; D055672:Static Electricity; D000071737:Stromal Interaction Molecule 1
chemical_list: D015220:Calcium Channels; D008565:Membrane Proteins; D009363:Neoplasm Proteins; D000071740:ORAI1 Protein; C510746:ORAI1 protein, human; C103530:STIM1 protein, human; D000071737:Stromal Interaction Molecule 1; D002118:Calcium
keywords: Conformational change; FRET; Orai1; STIM1; Store-operated calcium entry