Cotranslational Import Allows Some Polypeptides to Enter the ER as They Are Being Synthesized

Question 1

• cotranslational import into the ER is the…?

- proteins handled in this way are?

Answer 1

• Cotranslational import into the ER is the first step in the pathway for delivering newly synthesized proteins to various locations within the endomembrane system
• proteins handled in this way are synthesized on ribsomes that become attached to the ER shortly after translation begins

Question 2

The signal hypothesis: Blobel and Sabitini

• what did they suggest? what did it propose?
• what does the ER signal sequence do ?

Answer 2

• suggested the signal hypothesis, which proposed that proteins that move into the ER during synthesis possess an intrinsic signal, directing them to the ER
• The ER signal sequence directs the mRNA polypeptide complex to the rough ER surface

Question 3

Milstein et al.

• what did they study?
• what did they find?

Answer 3

• studied synthesis of small subunit (or light chain) of the protein immunoglobulin G
• evidence that a 20 amino acid sequence at the N-terminus of the light chain of immunoglobulin G is a signal sequence

Question 4

ER targeting of proteins

• what do other polypeptides destined for the ER contain?
• what are proteins containing the signal sequence referred to as?
• how long are the signal sequences and what 3 domains do they consist of?

Answer 4

• other polypeptides destined for the ER also contain the N-terminal sequence to target them to the ER
• proteins containing the signal sequences are referred to as preproteins
• the signal sequences are usually 15-30 amino acids long and consists of the three domain:
  1. a positively charged N-terminal region
  2. a central hydrophobic region
  3. a polar region adjoining the site where cleavage fro the mature protein will take place