Anoctamin 8 tethers endoplasmic reticulum and plasma membrane for assembly of Ca signaling complexes at the ER/PM compartment.

Communication and material transfer between membranes and organelles take place at membrane contact sites (MCSs). MCSs between the ER and PM, the ER/PM junctions, are the sites where the ER Ca sensor STIM1 and the PM Ca influx channel Orai1 cluster. MCSs are formed by tether proteins that bridge the opposing membranes, but the identity and role of these tethers in receptor-evoked Ca signaling is not well understood. Here, we identified Anoctamin 8 (ANO8) as a key tether in the formation of the ER/PM junctions that is essential for STIM1-STIM1 interaction and STIM1-Orai1 interaction and channel activation at a ER/PM PI(4,5)P-rich compartment. Moreover, ANO8 assembles all core Ca signaling proteins: Orai1, PMCA, STIM1, IP receptors, and SERCA2 at the ER/PM junctions to mediate a novel form of Orai1 channel inactivation by markedly facilitating SERCA2-mediated Ca influx into the ER. This controls the efficiency of receptor-stimulated Ca signaling, Ca oscillations, and duration of Orai1 activity to prevent Ca toxicity. These findings reveal the central role of MCSs in determining efficiency and fidelity of cell signaling.