Sorting and processing of secretory proteins.

a common biosynthetic origin in the rough endoplasmic reticulum (RER), from where they are transported to the Golgi complex. It is in the trans-Golgi network (TGN) that proteins destined for the regulated secretory pathway will be sorted from those to be secreted via the constitutive pathway. Both of these pathways involve vesicular transfer to the plasma membrane followed by the secretory event itself, exocytotic discharge of vesicle contents. In many instances, secretory proteins, even after cleavage of the signal peptide in the lumen of the RER, are in the form of a higher-molecular-mass precursor or proprotein. Conversion of proprotein to protein occurs in both the regulated and the constitutive pathways, but in different compartments and as a result of the action of different endoproteases. Sorting in the TGN and conversion of proproteins will be the major focus of this review.