Signal Peptide - Co-translational Versus Post-translational Translocation

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). SRP then halts further translation 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). 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.

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 turns pumps the protein through the translocon. Although post-translational translocation is known to occur in eukaryotes, it is poorly understood. It is however known that in yeast post-translational translocation requires two membrane-bound proteins, Sec62 and Sec63.