Translocons

Translocons are protein-conducting channels embedded in biological membranes that mediate the translocation of polypeptides across membranes or their integration into membrane environments. They are essential for secretion, organellar protein import, and membrane protein assembly in all domains of life.

In eukaryotes, the best characterized translocon is the Sec61 complex in the endoplasmic reticulum (ER). Sec61 forms a central pore through which nascent polypeptides pass. It interacts with ribosomes to drive co-translational translocation and with auxiliary factors for post-translational pathways. Signal peptides or transmembrane segments guide polypeptides into the channel, and signal peptides are typically cleaved after translocation. Chaperones such as BiP in the ER lumen assist folding and, with ATP hydrolysis, can help pull the polypeptide into the lumen.

In other organelles and bacteria, translocons fulfill similar roles with different machineries. Mitochondria utilize the TOM complex in the outer membrane and the TIM complexes in the inner membrane to import nuclear-encoded proteins, aided by membrane potential and mitochondrial chaperones. Chloroplasts use the TOC/TIC complexes for protein import. In bacteria, the SecYEG complex forms the core channel of the plasma membrane, driven by the SecA ATPase; some proteins are secreted post-translationally, while others are translocated co-translationally. Additional pathways, such as the Tat system, handle folded proteins with distinct translocons.

Mechanistically, translocons provide a regulated pore with a lateral gate that can insert hydrophobic segments into the lipid bilayer. Energy sources such as ATP and, when relevant, membrane potential, power translocation and protein maturation across cellular membranes.