Editing Signal peptide (section)

==Co-translational versus post-translational translocation==
In both prokaryotes and eukaryotes signal sequences may act co-translationally or post-translationally.

The co-translational pathway is initiated when the signal peptide emerges from the [[ribosome]] and is recognized by the [[signal-recognition particle]] (SRP).<ref>{{cite journal | vauthors = Walter P, Ibrahimi I, Blobel G | title = Translocation of proteins across the endoplasmic reticulum. I. Signal recognition protein (SRP) binds to in-vitro-assembled polysomes synthesizing secretory protein | journal = The Journal of Cell Biology | volume = 91 | issue = 2 Pt 1 | pages = 545–50 | date = November 1981 | pmid = 7309795 | pmc = 2111968 | doi = 10.1083/jcb.91.2.545 }}</ref> SRP then halts further translation (translational arrest only occurs in Eukaryotes) and directs the signal sequence-ribosome-mRNA complex to the [[SRP receptor]], which is present on the surface of either the plasma membrane (in prokaryotes) or the ER (in eukaryotes).<ref>{{cite journal | vauthors = Gilmore R, Blobel G, Walter P | title = Protein translocation across the endoplasmic reticulum. I. Detection in the microsomal membrane of a receptor for the signal recognition particle | journal = The Journal of Cell Biology | volume = 95 | issue = 2 Pt 1 | pages = 463–9 | date = November 1982 | pmid = 6292235 | pmc = 2112970 | doi = 10.1083/jcb.95.2.463 }}</ref> Once membrane-targeting is completed, the signal sequence is inserted into the translocon. Ribosomes are then physically docked onto the cytoplasmic face of the translocon and protein synthesis resumes.<ref>{{cite journal | vauthors = Görlich D, Prehn S, Hartmann E, Kalies KU, Rapoport TA | title = A mammalian homolog of SEC61p and SECYp is associated with ribosomes and nascent polypeptides during translocation | journal = Cell | volume = 71 | issue = 3 | pages = 489–503 | date = October 1992 | pmid = 1423609 | doi = 10.1016/0092-8674(92)90517-G | s2cid = 19078317 }}</ref>

The post-translational pathway is initiated after protein synthesis is completed. In prokaryotes, the signal sequence of post-translational substrates is recognized by the [[SecB]] [[chaperone protein]] that transfers the protein to the [[SecA]] ATPase, which in turn pumps the protein through the translocon. Although post-translational translocation is known to occur in eukaryotes, it is poorly understood. It is known that in yeast post-translational translocation requires the translocon and two additional membrane-bound proteins, [[TLOC1|Sec62]] and [[SEC63|Sec63]].<ref name="Panzner1995">{{cite journal | vauthors = Panzner S, Dreier L, Hartmann E, Kostka S, Rapoport TA | title = Posttranslational protein transport in yeast reconstituted with a purified complex of Sec proteins and Kar2p | journal = Cell | volume = 81 | issue = 4 | pages = 561–70 | date = May 1995 | pmid = 7758110 | doi = 10.1016/0092-8674(95)90077-2 | s2cid = 14398668 | doi-access = free }}</ref>