Transmembrane protein: does signal peptide always form a loop?
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.


http://web.uconn.edu/mcb380/fig12-45a.GIF

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.