AD: Ribosome-Associated ER Transport

• integral membrane proteins (with trans-membrane domain)

• secreted proteins

• some housekeeping proteins

in totlal around 40% of the proteome

• ribosome

• signal sequence (SS) of the protein

• signal recognition particle (SRP)

• signal receptor (SR)

• translocon (SecYEG, pore of the ER)

• Signal sequence (SS): certain hydrophobic sequence at the N-terminus of the peptide chain that is being synthesized.

  • Some proteins have additional TMD (trans-membrane domain) to be inserted into the membrane.

• The signal sequence is recognized by the signal recognition particle (SRP) and the translation is stalled.

• The ribosome-mRNA-SRP complex is transported to a pore (translocon) of the ER, recognized by a signal receptor (SR)

Process of recognition and translocation:

a: Cargo recognition. SRP interacts with both sides of the ribosomal exit channel. Its fingerloop interacts with the signal sequence of the nascent protein.

b: Early complex. Upon GTP binding, SRP’s NG domains hetero-dimerize. Formerly unstructured domains become structured, the fingerloop wraps around the signal sequence more closely.

c: Closed complex. The NG-heterodimer domains detach from the ribosome. The unstructured A domain becomes available.

d: Activated complex. The unstructured A domain anchors the complex to the membrane. GTP hydrolysis is triggered, the fingerloop opens, and the signal sequence is handed over to translocon.

e: Translocation. The nascent peptide chain is detached from the ribosome. Translocation/Insertion into the membrane through translocon activity takes place.

• ribosomal translation (“pushing” of the protein)

• folding inside the ER prevents “backsliding” of the protein, pulling force