The EMC acts as a chaperone for membrane proteins.

Structure formation of membrane proteins is error-prone and thus requires chaperones that oversee this essential process in cell biology. The ER membrane protein complex (EMC) is well-defined as a transmembrane domain (TMD) insertase. In this study, we characterize an additional chaperone function of the EMC. We use interactomics and systematic studies with model proteins to comprehensively define client features for this EMC chaperone mode. Based on this data, we develop a machine learning-based tool for client prediction. Mechanistically, our study reveals that the EMC engages TMDs via its EMC1 subunit and modulates their orientation within the lipid bilayer. Productive TMD assembly reduces binding to the EMC chaperone site. Taken together, our study provides detailed insights into an EMC chaperone function, further establishing the role of the EMC as a multifunctional molecular machine in membrane protein biogenesis.