Signal hypothesis:

A classical secretory protein differs from a cytosolic protein through having a sequence about 13-35 amino acids long at its N-terminal end known as a signal peptide or signal sequence. The signal peptides of various secretory proteins differ  in amino acid sequence but there are  some common features, for instance the center of the sequence commonly consists of 10-15 hydrophobic  amino acids. An signal hypothesis was proposed from early work in this area and predicted in which the signal peptide  directs  the secretory  protein  to the RER  membrane  and  so goals  the protein to cross into the RER lumen and be exported.  The signal hypothesis has been defined to apply to protein secretion in plant, animal and bacterial cells. A simplified edition of the mechanism is shown in Figure.

The mRNA for the secretory protein binds to a free cytoplasmic protein and ribosome synthesis starts.  The first category of the protein made is the N-terminal signal peptide. A SRP (signal recognition particle), that is a complex of a 7S RNA  and  six  proteins will  binds  to  the  signal  peptide  and  stops  additional  protein synthesis. This ends the secretory protein from being released prematurely into the cytosol. The ribosome-mRNA- signal recognition particle complex now binds to a signal recognition particle receptor, a protein on the surface of the rough endoplasmic reticulum. The rough endoplasmic reticulum membrane also contains a ribosome receptor protein associated with a protein translocon. In a concerted sequence of reactions the ribosome is held tightly through the ribosome receptor protein and the signal recognition particle binds to the signal recognition particle receptor and is released from the signal peptide, translation now continues once more the nascent polypeptide passing through a pore in the membrane created through the translocon. As it passes by the pore the signal peptide is cleaved off through a signal peptidase on the lumenal face of the rough endoplasmic reticulum and degraded and releasing the rest of the protein into the lumen. The protein is then transported by the Golgi to the cell exterior as described above. Since transport across the rough endoplasmic reticulum membrane occurs during protein synthesis, the procedure is said to be co-translational.  The released signal recognition particle is cycled via its receptor ready for binding to another signal peptide (the signal recognition particle cycle).