A functional chaperone condensate in the endoplasmic reticulum

Abstract

One third of eukaryotic proteins are processed within the endoplasmic reticulum (ER) to obtain their correct structure1-3. This function is ensured by a network of molecular chaperones that recognizes client proteins and assists their folding4,5. How the ER chaperones organize in a supramolecular manner to exert their cooperativity has, however, remained unclear. Here, we report the discovery of a multi-chaperone condensate in the ER lumen, which is formed around the chaperone PDIA6 during protein folding homeostasis. We resolve the structure of PDIA6 and the mechanism underlying its condensate formation at the atomic and cellular level. We find that the multivalency required for PDIA6 phase separation is created by two specific interfaces. One of these interfaces is regulated by a Ca2+-dependent molecular switch, allowing dynamic adaptations to ER stress. The PDIA6 condensates recruit further chaperones, in particular Hsp70 BiP, J-domain protein ERdj3, disulfide isomerase PDIA1 and Hsp90 Grp94, which are the essential components of the early folding machinery. The chaperone condensates enhance folding of client proteins, which we show exemplarily on proinsulin, and prevent protein misfolding in the ER lumen. The PDIA6-scaffolded chaperone condensates hence provide the functional basis for spatial and temporal coordination of the dynamic ER chaperone network.