Most polypeptides synthesized on the rough endoplasmic reticulum are glycosylated, that is, they have sugar residues added to them, as soon as the growing polypeptide chain enters the lumen of the endoplasmic reticulum. In a process called N-glycosylation a premade oligosaccharide composed of two N-acetyl glucosamines, then nine mannoses, and then three glucoses is added to an asparagine residue by the enzyme oligosaccharide transferase. The three glucose residues are subsequently removed, marking the protein as ready for export from the endoplasmic reticulum to the Golgi apparatus.
The stacks of the Golgi apparatus (Fig. 10.8) have distinct polarity that reflects the passage of proteins through the organelle. The asymmetry of the stack is reflected in the cis face
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