Detailed Notes on Protein Translocation and Membrane Protein Maturation

Signal Sequence and Protein Translocation

Signal Sequence: A short peptide sequence that directs the transport of the growing polypeptide chain to the endoplasmic reticulum (ER), indicating where the protein should be synthesized and folded within the cell.
Sequence Emergence: The growing polypeptide chain emerges from the ribosome and interacts with the translocation machinery of the ER.
Closed Translocation Channel: Refers to the non-open state of the protein translocator channel through which the polypeptide chain is translocated into the ER lumen.
Cleaved Signal Peptide: Once the signal sequence is recognized, it is cleaved off by a specific enzyme called signal peptidase, resulting in a mature protein without the signal peptide.
End Result: This process leads to the formation of a mature soluble protein within the ER lumen.

Hydrophobic Start-Transfer Sequence: A sequence that helps initiate the embedding of the growing polypeptide chain into the membrane.
Stop-Transfer Sequence: A hydrophobic sequence that signals the translocator to halt the transfer of the protein chain into the ER lumen, allowing for the protein to become a transmembrane protein.
Protein Translocator: A complex membrane structure that facilitates the translocation of polypeptides into the ER.
Mature Single-Pass Transmembrane Protein: A type of protein that, following translocation, spans the membrane once, characterized by having an NH₂ (amino) terminus in the cytosol and a COOH (carboxyl) terminus in the ER lumen.
Signal Peptidase Action: Ensures that the signal peptide is cleaved correctly to yield the functional protein within the membrane.

Hydrophobic Start-Transfer Sequence: Also crucial for the insertion of proteins, it not only initiates translocation but also defines the orientation of the protein in the membrane.
Double-Pass Transmembrane Protein: A protein that spans the membrane twice, featuring two hydrophobic sequences that interact with the translocator channel.
Mechanism: The stop-transfer sequences allow certain portions of the polypeptide to exit into the lipid bilayer, defining a double-pass orientation where parts of the polypeptide remain embedded within the membrane while others are oriented towards the cytosol or ER lumen.
Outcome: The resulting mature double-pass protein has both NH₂ and COOH termini located in specific cellular compartments based on the arrangement of the start-transfer and stop-transfer sequences.