Fig. 4: Model of TRAP function in protein translocation across the ER membrane.

a, Schematic of the proposed function of the TRAP complex. The interplay of NAC and SRP initiates the targeting of the translating ribosome carrying an ER client to the ER membrane. Interaction with the SRP receptor at the ER membrane then initiates the transfer of the ribosome to Sec translocon. The tethered ribosome interacting TRAPα anchor could facilitate the transfer by attaching the ribosome near the Sec translocon. Once the handover has occurred, the TRAP-RNC-Sec61 complex is additionally stabilized by the TRAPγ finger on the ribosome as well as the TRAPα lumenal loop interacting with the translocon. In this conformation, the TRAP complex positions a hydrophobic cradle-like lumenal domain directly at the exit of the Sec61 pore, which could act as a chaperone for the incoming nascent chain. b, Schematic depicting the binding sites of SRP, NAC and Sec translocon on the ribosome exit tunnel region. The binding site of the TRAP anchor and NAC anchor are indicated. Binding site of TRAP finger is indicated with a dashed line. The position of the ribosome polypeptide tunnel exit is indicated with an asterisk.