Intracellular Transport of Proteins and Protein Modification

Abstract

Extensive transport of proteins takes place between different intracellular compartments. Protein synthesized for export or for insertion into a number of membraneous compartments are transferred from the endoplasmic reticulum to different parts of the vesicular space where modifications may take place. Various sugars are added by enzymes which first recognize defined structures in the polypeptide chain and then structures in the growing oligosaccharide chains. The different additions take place in different vesicular compartments. Thus sugars like N-acetyl glucosamine present in the proximal part of the oligosacharide chain, are added in the endocytic reticulum, while the more distal sugars, like galactose and sialic acid, are added in the Golgi apparatus. Subsequently the proteins are transported in vesicles to their final target or, in the case of export proteins, to the cell surface where they are released.