Fig. 1: Three modes of terminal TMD insertion for multipass membrane proteins.

TMDs located within ~50 amino acids of the stop codon of a multipass membrane protein will be partially or completely inside the ribosome exit tunnel at the time of translation termination. This means that they will necessarily be inserted by one of three post-translational mechanisms depending on the preceding membrane domain and intervening loop. a, When the penultimate TMD is followed by a long (more than 50 amino acids) translocated loop, its translocation will be Sec61-dependent⁸. Hence, the loop will already be threaded through the Sec61 channel by the time of termination (left diagram). The terminal TMD inside the ribosome will then necessarily enter Sec61 (middle diagram), from where it presumably accesses the membrane through Sec61’s lateral gate. b, When the final two TMDs are closely spaced, neither one can be inserted co-translationally because the tether downstream of the penultimate TMD is too short to engage Sec61 at the time of termination (left diagram). Therefore, both TMDs will be released and inserted post-translationally by an unknown mechanism. c, When the final TMD is located near a translocated C terminus, the TMD and tail are mostly or entirely inside the ribosome at the time of termination (left diagram). Their post-translational insertion mechanism is also unknown. Note that the machinery involved in the insertion and chaperoning of earlier TMDs is not shown for simplicity.