Post Translational Modifications -

Post Translational Modifications - Moving Proteins into Membranes Part 2
Membrane and soluble proteins synthesized on the rough ER undergo four principal modifications;

1) Glycosylation - covalent addition of one or more carbohydrate chains attached to OH group of Serine or Threonine (O-linked oligosaccharides) or the amide nitrogen of Asparagine (N-linked oligosaccharides) in the ER or Golgi complex

2) Disulfide bond formation in the ER

3) Folding of polypeptide chains and assembly of multisubunit proteins in the ER

4) Proteolytic cleavages in the ER, Golgi complex, and secretory vesicles

These modifications promote folding of secretory proteins into their final conformation and may add structural stability to proteins in the extracellular environment. Glycosylation also allows the production of chemically distinguished molecules on the cell surface that plays vital roles in cell-to-cell adhesion and communication, and accounts for the majority of modifications to proteins. Proteins with attached carbohydrates are known as glycoproteins. O-linked chains typically contain only one to four carbohydrate units added onto the protein through enzymes known as glycosyltransferases, found in the lumen of the Golgi complex.

More commonly N-linked oligosaccharides are larger and more complex, and biosynthesis starts in the rough ER. A preformed oligosaccharide containing 14 residues, with a conserved structure found amongst eukaryotic cells, containing three glucose (Glc), nine mannose (Man), and two N-acetylglucosamine molecules (GlcNAc), is attached to a membrane anchor called Dolichol phosphate.