Posttranslational Modification

If a protein is going to be used in the cytosol (for example, the enzymes of glycolysis), it is likely to be made by free ribosomes in the cytosol. If it is going to be packaged into a lysosome or secreted from the cell, however, its signal peptide causes the entire polyribosome to migrate to the rough ER and dock on its surface. Assembly of the amino acid chain is then completed on the rough ER and the protein is sent to the Golgi complex for final modification. Thus, we turn to the functions of these organelles in the modification, packaging, and secretion of a protein (fig. 4.10).

Saladin: Anatomy & I 4. Genetics and Cellular I Text I © The McGraw-Hill

Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition

Chapter 4 Genetics and Cellular Function 139

double helix

DC

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M

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1 T A C

C G C

C C T 1

T G C G T A

C T C

A C T 1

coding strand

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A U G

G C G

G G A

A C G C A U

G A G

U G A

Codons of mRNA

"Start"

"Stop"

UAC 1

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CCU 1

UGC

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Anticodons of tRNA

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Amino acids

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Figure 4.9 Relationship of a DNA Base Sequence to Protein Structure. (1) DNA. (2) A series of base triplets in the coding strand of DNA. (3) The corresponding codons that would be in an mRNA molecule transcribed from this DNA sequence. (4) Binding of mRNA to the complementary anticodons of six tRNA molecules. (5) The amino acids bound to these tRNAs. (6) Linkage of the amino acids into the peptide that was encoded in the DNA.

Figure 4.9 Relationship of a DNA Base Sequence to Protein Structure. (1) DNA. (2) A series of base triplets in the coding strand of DNA. (3) The corresponding codons that would be in an mRNA molecule transcribed from this DNA sequence. (4) Binding of mRNA to the complementary anticodons of six tRNA molecules. (5) The amino acids bound to these tRNAs. (6) Linkage of the amino acids into the peptide that was encoded in the DNA.

When a protein is produced on the rough ER, its signal peptide threads itself through a pore in the ER membrane and drags the rest of the protein into the cisterna. Enzymes in the cisterna then remove the signal peptide and modify the new protein in a variety of ways—removing some amino acids segments, folding the protein and stabilizing it with disulfide bridges, adding carbohydrate moieties, and so forth. Such changes are called posttranslational modification. Insulin, for example, is first synthesized as a polypeptide of 86 amino acids. In posttranslational modification, the chain folds back on itself, three disulfide bridges are formed, and 35 amino acids are removed. The final insulin molecule is therefore made of two chains of 21 and 30 amino acids held together by disulfide bridges (see fig. 17.15).

When the rough ER is finished with a protein, it pinches off clathrin-coated transport vesicles. Like the address on a letter, clathrin may direct the vesicle to its destination, the Golgi complex. The Golgi complex removes the clathrin, fuses with the vesicle, and takes the protein into its cisterna. Here, it may further modify the protein, for example by adding carbohydrate to it. Such modifications begin in the cisterna closest to the rough ER. Each cisterna forms transport vesicles that carry the protein to the next cisterna, where different enzymes may further modify the new protein.

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