Translocon

The translocon (commonly known as a translocator or translocation channel) is a complex of proteins associated with the translocation of polypeptides across membranes. In eukaryotes the term translocon most commonly refers to the complex that transports nascent polypeptides with a targeting signal sequence into the interior (cisternal or lumenal) space of the endoplasmic reticulum (ER) from the cytosol. This translocation process requires the protein to cross a hydrophobic lipid bilayer. The same complex is also used to integrate nascent proteins into the membrane itself (membrane proteins). In prokaryotes, a similar protein complex transports polypeptides across the plasma membrane or integrates membrane proteins. Bacterial pathogens can also assemble other translocons in their host membranes, allowing them to export virulence factors into their target cells.

The bacterial translocation channel is formed by a hetero-trimeric protein complex called SecYEG. It consists of the subunits SecY, SecE, and SecG. The structure of the idle archaeal homolog of this complex has been determined by X-ray crystallography. (ref. #4 is incomplete). The large SecY subunit (alpha-subunit) consists of two halves, trans-membrane segments 1-5 and trans-membrane segments 6-10. They are linked at the extracellular side by a loop between trans-membrane segments 5 and 6. SecY can open laterally at the front (lateral gate). SecE (gamma-subunit) is a single spanning membrane protein in most species. It sits at the back of SecY, wrapping around the two halves of SecY. SecG (beta-subunit) is not essential. Its sits on the side of SecY and makes only few contacts with it. In a side view, the channel has an hourglass shape, with a cytoplasmic funnel that is empty, and an extracellular funnel that is filled with a little helix, called the plug. In the middle of the membrane is a construction, formed from a pore ring of six hydrophobic amino acids that project their side chains inwards. During protein translocation, the plug is moved out of the way, and a polypeptide chain is moved from the cytoplasmic funnel, through the pore ring, the extracellular funnel, into the extracellular space. Hydrophobic segments of membrane proteins exit sideways through the lateral gate into the lipid phase and become membrane-spanning segments.

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