Stress-induced phase separation of ERES components into Sec bodies precedes ER exit inhibition in mammalian cells.

Abstract

Phase separation of ER-exit-sites (ERES) components into membraneless compartments, the Sec bodies, occurs in Drosophila cells upon specific cellular stressors, i.e., salt stress and amino acid starvation, and their formation is linked to the inhibition of the early secretory pathway. Here, we show Sec bodies also form in secretory mammalian INS-1 cells upon the same stress. These reversible and membraneless structures are positive for ERES components, including both isoforms of Sec16 (A and B) and COPII subunits. We find that Sec16A, but not Sec16B, is a driver for Sec body formation. We show that the coalescence of ERES components into Sec bodies occurs by fusion, in line with their liquid-droplet properties. Lastly, we demonstrate that stress-induced ER-exit inhibition is a consequence of the significant coalescence of Sec16A into Sec bodies, leading to its depletion from ERES that become non-functional. Stress relief causes an immediate dissolution of Sec bodies and the concomitant restoration of protein exit from the ER. We propose a model in which dynamic conversion between ERES and Sec body assembly, driven by Sec16A, regulates protein exit from the ER during stress and upon stress relief in mammalian cells, thus providing a conserved pro-survival mechanism in response to stress.