Транслокон: Полипептидтерді мембрана арқылы тасымалдау механизмі

The translocon (also 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 (inner) plasma membrane or integrates membrane proteins. In either case, the protein complex are formed from Sec proteins (Sec: secretory), with the heterotrimeric Sec61 being the channel. In prokaryotes, the homologous channel complex is known as SecYEG.

The translocation channel is a hetero trimeric protein complex called SecYEG in prokaryotes and Sec61 in eukaryotes. It consists of the subunits SecY, SecE, and SecG. The structure of this channel, in its idle state, has been solved by X ray crystallography in archaea. SecY is the large pore subunit. In a side view, the channel has an hourglass shape, with a funnel on each side. The extracellular funnel has a little "plug" formed out of an alpha helix. 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. In eukaryotes, Sec61 forms a complex with the oligosaccharyl transferase complex, the TRAP complex, and the membrane protein TRAM (possible chaperone). For further components, such as signal peptidase complex and the SRP receptor it is not clear to what extent they only associate transiently to the translocon complex.

The channel allows peptides to move in either direction, so additional systems in the translocon are required to move the peptide in a specific direction. There are three types of translocation: cotranslational translocation that happens as translation happens, and two types of post translational translocation that happens after translation, each seen in eukaryotes and bacteria. While eukaryotes unfold the protein with BiP and use other complexes to transport the peptide, bacteria use the SecA ATPase.

In co translational translocation, the translocon associates with the ribosome so that a growing nascent polypeptide chain is moved from the ribosome tunnel into the SecY channel. 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. 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. In eukaryotes, proteins due to be translocated to the endoplasmic reticulum are recognized 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 SecA ATPase uses a "push and slide" mechanism to move a polypeptide through the channel. 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. SecA then grabs a further section of the peptide to repeat the process. However, the structure of Sec61 does not support this view and several different proteins have been suggested to be responsible for transport from the ER lumen into the cytosol.