Protein Targeting and Processing

It is the ER that provides the cell with the mechanism for segregating newly synthesized proteins that are to remain in the cytosol and those destined for the plasma membrane, storage in lysosomes, or secretion from the cell. 

The signal that directs a ribosome with its nascent protein to the ER membrane is a sequence of amino acids near the amino terminus of its nascent polypeptide chain. In a secretory protein, these signal sequences are present 

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. 

Other kinds of modifications may be necessary to convert a newly synthesized protein to its biologically active form. The N-formyl group of the initiating methionine in prokaryotes is removed by a deformylase. A methionine amino-peptidase removes the initiating residue in many eukaryotic proteins. Other posttranslational modifications may include acetylation, amidation, hydroxy lation, methylation, phosphorylation, and sulfation of specific amino acid resi- 

Proteolytic conversion of inactive zymogens to active enzymes was noted in Chapter 5. Other examples of posttranslational proteolytic cleavages include the conversions of proalbumin to albumin (Chapter 7), of preprocollagen to collagen (Chapter 8), and of preproinsulin to insulin (Chapter 16) and the activation of the compounds of the blood-clotting cascade (Chapter 7). 

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