This review examines protein targeting to the endoplasmic reticulum (ER) in yeast, focusing on two parallel translocation pathways that converge at a common pore. The signal recognition particle (SRP) mediates co-translational targeting of nascent proteins with hydrophobic signal sequences, directing them to the Sec61 complex translocon. In contrast, proteins with less hydrophobic signal sequences follow a post-translational SRP-independent pathway utilizing the Sec62/63/71/72 complex and luminal chaperones like Kar2p. The Sec61 complex, a tripartite channel conserved across eukaryotes, forms an oligomeric ring structure that acts as the central translocation pore. Recent structural and biophysical studies reveal this channel is surprisingly large (40-60 Å diameter), potentially accommodating partially folded polypeptides. The channel appears dynamically gated at both cytosolic and luminal faces through mechanisms involving the ribosome and the signal sequence itself. Components of this translocon also participate in dislocation, the retrograde transport of misfolded proteins from the ER to the cytosol for degradation. Membrane protein insertion involves lateral release of hydrophobic domains into the lipid bilayer through mechanisms still being elucidated, with the ribosome apparently playing a regulatory role in mode switching between translocation and insertion.

Key findings

• Two parallel protein-targeting pathways to the ER (SRP-dependent co-translational and SRP-independent post-translational) merge at the Sec61 complex translocon.
• The translocation channel is large (40-60 Å diameter) and may permit folding of nascent polypeptides during translocation, with dynamic gating mechanisms at both cytosolic and luminal faces.
• The Sec61 complex is evolutionarily conserved across eukaryotes and shares structural similarity with bacterial SecY/E/G translocase, forming oligomeric rings of 2-4 trimers.
• Signal sequence hydrophobicity determines which targeting pathway a precursor follows, with less hydrophobic sequences utilizing the post-translational route.
• The ribosome plays an active regulatory role in gating channel opening/closing and may coordinate mode switching during membrane protein insertion.