Golgi complex protein glycosylation

Protein glycosylation occurs in the endoplasmic reticulum (ER) and Golgi compartments of the cell and involves a complex series of enzymatic reactions catalysed by membrane-bound glycosyltransferases and glyco-sidases. The two main types of protein glycosylation have very different biosynthetic pathways. 

Processing of proteins in the Golgi complex includes sorting and glycosylation of membrane proteins and secretory proteins 148 Proteins and lipids move through Golgi cisternae from the cis to the trans direction 148... 

Secretory proteins have an N-terminal signal peptide which targets the protein to be synthesized on the rough endoplasmic reticulum (RER). During synthesis it is translocated through the RER membrane into the lumen. Vesicles then bud off from the RER and carry the protein to the Golgi complex, where it becomes glycosylated. Other vesicles then carry it to the plasma membrane. Fusion of these transport vesicles with the plasma membrane then releases the protein to the cell exterior. 

Many secretory and membrane proteins are modified in the lumen of the ER. Protein glycosylation, the addition of covalently bound oligosaccharide chains, is a common reaction in the ER lumen and the Golgi complex. Most proteins in the ER lumen destined for secretion from the cell or for transport to other intracellular sites are glycoproteins. The carbohydrate content of glycoproteins can vary from 0.5 to 80% or more of the glycoprotein mass. Glycoprotein structures were described in Chapter 9, and their biosynthesis are covered in Chapter 20. 

Protein Glycosylation Takes Place in the Lumen of the Endoplasmic Reticulum and in the Golgi Complex... 

Transport Vesicles Carry Proteins from the Endoplasmic Reticulum to the Golgi Complex for Further Glycosylation and Sorting... 

Shestakova A, Zolov S, Lupashin V. COG complex-mediated recycling of Golgi glycosyltransferascs is essential for normal protein glycosylation. TraBic 2006 7(2) 191-204. 

The RER is involved in the synthesis of certain proteins. Ribosomes attached to the membranes of the RER give them their rough appearance. Proteins produced on these ribosomes enter the lumen of the RER, travel to the Golgi complex in vesicles, and are subsequently either secreted from the cell, sequestered within membrane-enclosed organelles such as lysosomes, or embedded in the plasma membrane. Posttranslational modifications of these proteins, such as the initiation of N-linked glycosylation and the addition of GPl anchors, occur in the RER. In contrast, proteins encoded by the nucleus and found in the cytosol, peroxisomes, or mitochondria are synthesized on free ribosomes in the cytosol and are seldom modified by the attachment of oligosaccharides. 

---