role of endoplasmic reticulum in protein modification and packaging:
Newly synthesized polypeptides in the membrane and lumen of the ER undergo five principal modifications before they reach their final destinations:
1.Formation of disulfide bonds
2.Proper folding
3.Addition and processing of carbohydrates
4.Specific proteolytic cleavages
5.Assembly into multimeric proteins
The first two and the last of these modifications, which take place exclusively in the rough ER, are discussed in this section. Although addition of some carbohydrates and some proteolytic cleavages also occur in this organelle, many such modifications take place in the Golgi complex or forming secretory vesicles; we discuss these in later sections.
