Translocon

[1] 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.

A larger heptameric complex that includes the core trimeric protein and a tetramer is responsible for the transportation of a subset of polypeptides into the endoplasmic reticulum.

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.

The co-translational translocation process in eukaryotes involves SRP that guide nascent polypeptide chains to the translocon while they are still associated with the ribosome.

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 an unfolded string of amino acids, potentially driven by a Brownian Ratchet.

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.

[11] In bacteria, the same process is done by a "pushing" ATPase known as SecA, sometimes assisted by the SecDF complex on the other side responsible for pulling.

In the ATP-bound state, SecA interacts through a two-helix finger with a subset of amino acids in a substrate, pushing them (with ATP hydrolysis) into the channel.

The interaction is then weakened as SecA enters the ADP-bound state, allowing the polypeptide chain to slide passively in either direction.

[11] Translocators can also move polypeptides (such as damaged proteins targeted for proteasomes) from the cisternal space of the endoplasmic reticulum to the cytosol.