1
$\begingroup$

So I have read a few times that

b. SP probably forms loop not arrow. Loop enters channel (translocon) in membrane. SP loop is probably what opens (gates) the channel on the cytoplasmic side.

Source

And I was wondering why this would be the case? It seems much more complicated to me for the protein to loop around than fior the signal peptide to just embed itself in the translocator pore and the rest of the polypeptide chain be synthesised in the cytoplasmic side.

Then if the protein was meant to pass through the membrane again, there wold be another start-transfer signal sequence (so the two first signal sequences would be two start-transfer sequences rather than a start transfer and a stop transfer).

$\endgroup$
2
$\begingroup$

I think perhaps a picture might help, also see some lecture notes here, where I pulled the pictures from: in short, if you have an unbroken chain, you can't stick it through a hole except as a loop unless you start at the very end, and often you don't want to do that.

Signal sequences can be anywhere on the protein. That portion is the part of the peptide that binds to the translocator pore in the membrane. That portion has to stay bound for the process to continue: it isn't possible to just "feed through" the growing polypeptide without securing it somehow, and that's precisely what the signal sequence is doing by interacting with the translocon.

Membrane protein synthesis http://web.uconn.edu/mcb380/fig12-45a.GIF Membrane protein synthesis 2 http://web.uconn.edu/mcb380/fig12-45b.GIF

This process also allows for all sorts of arrangements: sending the whole protein through, or just the N- or C- end, or having multiple signal sequences to create multiple transmembrane domains. Especially in the latter case, there really isn't any other way to do it except by forming some loop someplace, unless you were to actually ligate separate polypeptides together, which would be quite messy and difficult and require additional specialized enzymes: much more complicated than a nice, tidy loop.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.