Insertion of Multi-pass Transmembrane Proteins in the RER

Multipass transmembrane proteins are inserted through the Sec61 translocon channel during translation. The N-terminal signal sequence initiates translocation, and when hydrophobic domains are encountered, the translocon pauses and opens its lateral gate to embed each domain into the lipid bilayer. Translation resumes between domains, creating alternating transmembrane and cytosolic regions until the protein is complete.

The number of transmembrane domains determines terminal orientation. Proteins with odd-numbered transmembrane domains position their N and C termini on opposite sides of the membrane, while those with even-numbered domains align both termini on the same side. Additionally, the positioning of positive residues before or after the first transmembrane domain influences whether the N-terminal faces the cytosol or ER lumen.

The signal recognition particle (SRP) and its receptor (SR) initiate translocation of the first transmembrane domain through the ER membrane. However, their involvement is limited to the initial domain insertion. Threading of subsequent transmembrane domains is independent of the SRP-SR complex and depends primarily on the hydrophobicity of each translated domain and the ribosome-translocon assembly.

Accessory complexes like TRAP and TRAM interact with the Sec61 translocon to facilitate accurate folding, insertion, and assembly of multipass transmembrane proteins. While the Sec61 translocon-ribosome assembly forms the core of protein translocation, these accessory complexes are essential because multipass proteins have complex biophysical features that require additional support for proper membrane integration.

Multiple factors affect multipass protein topology, including the length and hydrophobic profile of transmembrane domains, the distance between consecutive domains, and the length of extra-membrane loops. These biophysical characteristics determine how each domain interacts with the translocon and lipid bilayer, ultimately establishing the protein's final spatial arrangement and orientation within the ER membrane.

Multipass protein insertion is more complex because it requires multiple cycles of translocation pausing and resumption for each hydrophobic domain. Unlike single-pass proteins, multipass proteins must coordinate the sequential embedding of multiple transmembrane domains while maintaining proper topology. The interplay of domain hydrophobicity, loop lengths, and accessory protein complexes makes their insertion significantly more challenging than insertion of single-pass transmembrane proteins.

The N-terminal ER signal sequence acts as the start-transfer cue for initial polypeptide translocation down the Sec61 channel. Unlike single-pass proteins, multipass proteins typically lack a cleavable signal sequence and instead use their first hydrophobic transmembrane domain as the ER signal sequence. The signal peptidase complex cleaves this initial sequence on the ER membrane after translocation begins.