6.7.1 Polypeptide Destinations: Signal Peptides and ER Ribosomes

Question 1

note

Answer 1

• It takes less than a minute to produce an average-sized polypeptide, hundreds of amino acids long. Usually a single mRNA strand is used to make several copies of a polypeptide simultaneously, increasing the speed of polypeptide production further
• Picture (a) is a transmission electron micrograph (TEM) of polyribosomes.
• Illustration (b) is a schematic representation of polyribosomes. During translation, the first ribosome attaches to the 5’ initiation sequence of the mRNA molecule. As the ribosome moves in the 3’ direction, a new incoming ribosome attaches to the 5’ site. The result is a string of ribosomes in various stages of polypeptide synthesis.
• The synthesis of all proteins begins in the cytosol where free ribosomes translate mRNA molecules. After translation, some proteins are bound to the cytosol whereas some will enter the endomembrane system.
• The polypeptides of proteins destined for the endomembrane system are marked by signal peptides. The entire polypeptide-ribosome complex becomes embedded into the ER membrane.
• Once synthesis is complete, the polypeptide may be exported from the cell (if it is a secretory protein) or remain partially embedded in the ER membrane (if it is a membrane protein.)

Question 2

polyribosome

Answer 2

• an aggregation of several ribosomes attached to one mRNA molecule, increasing the efficiency of polypeptide synthesis

Question 3

Signal peptides

Answer 3

• sequences of amino acids on polypeptides that target a protein to the ER

Question 4

ER ribosomes

Answer 4

• Ribosomes that carry ER-bound proteins bind to a receptor site of the ER and become ER ribosomes. The polypeptide moves through the ER membrane and enters the ER compartment.

Question 5

Match the following terms with the appropriate descriptions and choose the correct order of answers.

Answer 5

• d, c, b, a

Question 6

According to the signal hypothesis,

Answer 6

• a signal sequence is required to tag proteins that are destined for components of the endomembrane system

Question 7

The synthesis of a polypeptide is occurring in the cytosol. If this protein is destined for mitochondria, chloroplasts or peroxisomes, the growing polypeptide strand

Answer 7

• continues to be synthesized in the cytosol

Question 8

Signal sequences on polypeptide chains usually are found

Answer 8

• only on the amino-terminal end of the chain