Secretory proteins move through the endoplasmic reticulum membrane via an aqueous, gated pore

The environment of secretory proteins undergoing translocation across the ER membrane was determined by incorporating fluorescent probes into nascent chains during translation. Dyes were positioned at various locations across the entire bilayer and inside the ribosome, and in each case the probes were in an aqueous milieu, as shown both by their fluorescence lifetimes and by collisional quenching of their fluorescence by iodide ions introduced into the ER lumen. The nascent chain therefore occupies an aqueous pore that spans the entire membrane. Since the pore is sealed off from the cytoplasm, cotranslational translocation is effected topographically. This pore is not open to the lumen after targeting is completed; it opens only after the nascent chain length reaches about 70 residues.