A coiled-coil clamp controls both conformation and clustering of stromal interaction molecule 1 (STIM1) [Signal Transduction]

October 23rd, 2014 by Fahrner, M., Muik, M., Schindl, R., Butorac, C., Stathopulos, P., Zheng, L., Jardin, I., Ikura, M., Romanin, C.

Store-operated Ca2+ entry, essential for the adaptive immunity, is initiated by the endoplasmic reticulum (ER) Ca2+ sensor STIM1. Ca2+ entry occurs through the plasma membrane resident Ca2+ channel Orai1 that directly interacts with the C-terminal STIM1 domain, named SOAR/CAD. Depletion of the ER Ca2+ store controls this STIM1/Orai1 interaction via transition to an extended STIM1 C-terminal conformation, exposure of the SOAR/CAD domain and STIM1/Orai1 co-clustering. Here we developed a novel approach termed FRET-derived Interaction in a Restricted Environment (FIRE) in an attempt to dissect the interplay of coiled-coil (CC) interactions in controlling STIM1 quiescent as well as active conformation and cluster formation. We present evidence of a sequential activation mechanism in the STIM1 cytosolic domains where the interaction between CC1 and CC3 segment regulates both SOAR/CAD exposure and CC3-mediated higher-order oligomerization as well as cluster formation. These dual levels of STIM1 auto-inhibition provide efficient control over the coupling to and activation of Orai1 channels.