The C-terminal tail guides assembly and degradation of membrane proteins

A large number of newly synthesized membrane proteins in the endoplasmic reticulum (ER) are assembled into multi-protein complexes, but little is known about the mechanisms required for either assembly or degradation of unassembled membrane proteins. We find that C-terminal transmembrane domains (C-TMDs) with shorter tails are inefficiently inserted into the ER membrane since the translation is terminated before they emerge from ribosomes. These TMDs of insufficient hydrophobicity are post-translationally retained by the Sec61 translocon, thus providing a time window for efficient assembly with TMDs from partner membrane proteins. The unassembled C-TMDs are slowly flipped into the ER lumen. While the luminal chaperone BiP captures flipped C-TMDs with long tails and routes them to the ER-associated quality control, C-TMDs with shorter tails are diffused into the nuclear membrane. Thus, our studies suggest that C-terminal tails harbor important biochemical features for both biosynthesis and quality control of membrane protein complexes.