Orchestration of secretory protein folding by ER chaperones

•Unique conditions for protein folding in the ER•Unique properties of secretory and membrane proteins•The major chaperone systems in the lumen•Specialization of ER chaperones to the folding needs in the lumen•Folding enzymes interact with the major ER chaperones.

The endoplasmic reticulum is a major compartment of protein biogenesis in the cell, dedicated to production of secretory, membrane and organelle proteins. The secretome has distinct structural and post-translational characteristics, since folding in the ER occurs in an environment that is distinct in terms of its ionic composition, dynamics and requirements for quality control. The folding machinery in the ER therefore includes chaperones and folding enzymes that introduce, monitor and react to disulfide bonds, glycans, and fluctuations of luminal calcium. We describe the major chaperone networks in the lumen and discuss how they have distinct modes of operation that enable cells to accomplish highly efficient production of the secretome. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.