The Cotranslational Integration of Membrane Proteins into the Phospholipid Bilayer Is a Multistep Process

During the cotranslational integration of a nascent protein into the endoplasmic reticulum membrane, the transmembrane (TM) sequence moves out of an aqueous pore formed by Sec61alpha, TRAM, and other proteins and into the nonpolar lipid bilayer. Photocross-linking reveals that this movement involves the sequential passage of the TM domain through three different proteinaceous environments: one adjacent to Sec61alpha and TRAM and two adjacent to TRAM that place different restrictions on TM domain movement. In addition, the TM sequence is not allowed to diffuse into the bilayer from the final TRAM-proximal site until translation terminates. Cotranslational integration is therefore linked to translation and occurs via an ordered multistep pathway at an endoplasmic reticulum site that is multilayered both structurally and functionally.