Translocon

The translocon (commonly known as a translocator) is the complex of proteins associated with the translocation of nascent polypeptides into the interior (cisternal or luminal) space of the endoplasmic reticulum (ER) from the cytosol. This process requires the protein to cross a hydrophobic lipid bilayer.

Translocators can also move polypeptides (such as damaged proteins targeted for proteasomes) from the cisternal space of the endoplasmic reticulum to the cytosol. This requires the expenditure of energy in the form of GTP.

Proteins due to be translocated to the endoplasmic reticulum are recognised by the signal-recognition particle (SRP), which halts translation of the polypeptide by the ribosome while it attaches the ribosome to the SRP receptor on the endoplasmic reticulum. This recognition event is based upon a specific N-terminal signal sequence that is in the first few codons of the polypeptide to be synthesised.

The current model of protein translocation is that the translocon (translocator) acts as a channel through the hydrophobic membrane of the endoplasmic reticulum (after the SRP has dissociated and translation is continued). The emerging polypeptide is threaded through the channel as a string of amino acids. Once translation is finished a signal peptidase cleaves off the short signal peptide from the nascent protein leaving the polypeptide free in the interior of the endoplasmic reticulum.

The translocon can also translocate and integrate membrane proteins in the correct orientation into the membrane of the endoplasmic reticulum. The mechanism of this process is not fully understood, but involves the recognition and processing by the translocon of hydrophobic stretches in the amino acid sequence which are destined to become transmembrane helices.

NB. An important translocon is known as Sec61.