Translocon
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The translocon is the complex of proteins associated with the translocation of nascent polypeptides into the cisternal space of the endoplasmic reticulum from the cytosol. This process requires the protein to cross a hydrophobic lipid bilayer.
Proteins due to be translocated to the endoplasmic reticulum are recognised by the signal-recognition particle, which transports the nascent protein to the translocon whilst it is still being synthesised by the ribosome. This recognition event is based upon a specific N-terminal signal sequence that is the first part of the protein to be synthesised and leave the ribosome. The translocon regulates the interaction of the ribosome with the endoplasmic reticulum membrane.
The current model of protein translocation is that the translocon acts as a channel through the hydrophobic membrane of the endoplamsic reticulum. Proteins are threaded through the channel as a string of amino acids, and only fold into their correct shape once in the cisternal space of the endoplasmic reticulum.
The translocon can also translocate and the integrate membrane proteins in the correct orientatio into the membrane of the endoplamsic 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 helixs.