Swing-out opening of stromal interaction molecule 1

Stromal interaction molecule 1 (STIM1) resides in the endoplasmic reticulum (ER) membrane and senses luminal calcium (Ca2+) concentration. STIM1 activation involves a large-scale conformational transition that exposes a STIM1 domain termed CAD/SOAR, - which is required for activation of the calcium channel Orai. Under resting cell conditions, STIM1 assumes a quiescent state where CAD/SOAR is suspended in an intramolecular clamp formed by the coiled-coil 1 domain (CC1) and CAD/SOAR. Here, we present a structural model of the cytosolic part of the STIM1 resting state using molecular docking simulations that take into account previously reported interaction sites between the CC1 alpha 1 and CAD/SOAR domains. We corroborate and refine previously reported interdomain coiled-coil contacts. Based on our model, we provide a detailed analysis of the CC1-CAD/SOAR binding interface using molecular dynamics simulations. We find a very similar binding interface for a proposed domain-swapped configuration of STIM1, where the CAD/SOAR domain of one monomer interacts with the CC1 alpha 1 domain of another monomer of STIM1. The rich structural and dynamical information obtained from our simulations reveals novel interaction sites such as M244, I409, or E370, which are crucial for STIM1 quiescent state stability. We tested our predictions by electrophysiological and Forster resonance energy transfer experiments on corresponding single-point mutants. These experiments provide compelling support for the structural model of the STIM1 quiescent state reported here. Based on transitions observed in enhanced-sampling simulations paired with an analysis of the quiescent STIM1 conformational dynamics, our work offers a first atomistic model for CC1 alpha 1-CAD/SOAR detachment.

Automated summary

STIM1 is a protein located in the endoplasmic reticulum (ER) membrane that senses the concentration of calcium ions and plays a role in the activation of calcium channels. Through molecular docking simulations, a structural model of the resting state of STIM1 was established, and the binding interface was analyzed using molecular dynamics simulations. Experimental results confirmed the validity of the model and provided insights into the conformational dynamics of the resting state of STIM1.