I used to think that signal peptides were necessary for a membrane protein to be displayed on the cell surface till I found that a GPR with multiple transmembrane structures have not a signal peptide. I am curious about the transmembrane process of this kind of protein.
From the SignalPrediction server http://www.cbs.dtu.dk/services/SignalP/ :
"NOTE (added May 2018): Remember, the presence or absence of a signal peptide is not the whole story about the localization of a protein! If you want to find out more about the sorting of your eukaryotic proteins, try the protein subcellular localization predictor DeepLoc."
and from the Wikipedia article on signal peptide https://en.wikipedia.org/wiki/Signal_peptide:
"Although most type I membrane-bound proteins have signal peptides, the majority of type II and multi-spanning membrane-bound proteins are targeted to the secretory pathway by their first transmembrane domain, which biochemically resembles a signal sequence except that it is not cleaved."
Dera Jingwei Huang,
please have a look at the great work from Gunnar von Heijne, who has published many papers on topology of transmembrane proteins and mechanisms on how they are integrated into cellular membranes. Here are some references to start with:
Article Membrane protein serendipity
Article Improved membrane protein topology prediction by domain assignments
Cheers Markus
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