Question

If a newly made polypeptide is to be secreted from a cell, what must occur before it is secreted?

A. Its signal sequence must target it to the plasma membrane, where it causes exocytosis.
B. Its signal sequence must target it to the ER, after which it goes to the Golgi.
C. Its signal sequence must be cleaved off before the polypeptide can enter the ER.
D. Its signal sequence must cause it to be encased in a vesicle as soon as it is translated.
E. It must be translated by a ribosome that remains free of attachment to the ER.

Answer 1

Before a polypeptide is secreted from a cell, its signal sequence targets it to the endoplasmic reticulum (ER), after which it undergoes modifications and targeting in the Golgi apparatus before being secreted.

Step-by-step explanation:

If a newly made polypeptide is to be secreted from a cell, several steps occur before it is secreted. The correct answer to what must occur is: Its signal sequence must target it to the endoplasmic reticulum (ER), after which it goes to the Golgi apparatus. This process is part of the protein targeting pathway and involves the following steps:

• The polypeptide begins to be synthesized by a ribosome and as it elongates, a signal sequence emerges.
• This signal sequence directs the ribosome to the ER membrane, leading the ribosome to bind to a translocation channel on the ER.
• The signal peptidase in the ER membrane catalyzes the co-translational hydrolysis of the signal peptide.
• Translation resumes, and the growing polypeptide folds inside the RER.
• After proper folding and possible modifications in the ER, the polypeptide is transported in a vesicle to the Golgi apparatus.
• In the Golgi, it undergoes further modifications and is sorted for its final destination.
• Finally, the polypeptide is packaged into a secretory vesicle that fuses with the plasma membrane to release the content outside the cell.