Molecular mechanisms of E-Syt-mediated stress resistance

Membrane contact sites (MCS) between the endoplasmic reticulum (ER) and the plasma membrane (PM) enable direct intermembrane exchange of signals and metabolites. The Extended Synaptotagmins (E-Syts) are an evolutionary conserved family of ER-PM tethers essential to maintain PM integrity under stress conditions. To investigate the underlying molecular mechanisms, we employed cellular reconstitution experiments in yeast and plants. We show that E-Syt-mediated stress tolerance relies on ER-PM MCS targeting, which requires the E-Syt N-terminal membrane anchor, a minimal set of two C2 domains and an SMP domain. C2 domains are sufficiently conserved that interspecies domains can sustain both PM localization and stress response. The role of the SMP domain in ER-PM localization is also conserved, but SMP function in stress resistance is species-specific. Furthermore, cryo-electron tomography uncovers a scaffolding role for the SMP domain in maintaining ER-PM distance, and in the formation of ER membrane peaks with extreme curvature that appear necessary for stress tolerance. Collectively, our findings reveal the individual and synergistic roles of all E-Syt modules in maintaining cellular homeostasis under stress.