XPI(4,5)P₂-Dependent and Ca²⁺-Regulated ER-PM interactions mediated by the extended synaptotagmins

Summary Most available information on endoplasmic reticulum (ER)-plasma membrane (PM) contacts in cells of higher eukaryotes concerns proteins implicated in the regulation of Ca²⁺ entry. However, growing evidence suggests that such contacts play more general roles in cell physiology, pointing to the existence of additionally ubiquitously expressed ER-PM tethers. Here, we show that the three extended synaptotagmins (E-Syts) are ER proteins that participate in such tethering function via C2 domain-dependent interactions with the PM that require PI(4,5)P₂ in the case of E-Syt2 and E-Syt3 and also elevation of cytosolic Ca²⁺ in the case of E-Syt1. As they form heteromeric complexes, the E-Syts confer cytosolic Ca²⁺ regulation to ER-PM contact formation. E-Syts-dependent contacts, however, are not required for store-operated Ca²⁺ entry. Thus, the ER-PM tethering function of the E-Syts (tricalbins in yeast) mediates the formation of ER-PM contacts sites, which are functionally distinct from those mediated by STIM1 and Orai1.