Question

Rough microsomes can be subjected to a "salt extraction" procedure in which a high salt concentration is used to remove membrane-associated ribosomes and peripheral proteins. Such salt-extracted microsomes are known to be translocation-incompetent, meaning that when present co-translationally in vitro, they fail to protect translated proteins from protease digestion. However, adding back an 11S particle (S is the sedimentation coefficient) purified from the salt-wash fraction is sufficient to restore the protein-translocation activity of the salt-extracted microsomes. Which of the following do you think is true regarding the 11S particle?

A. It is composed of 21 proteins.
B. It is a digestion product of ER-associated ribosomes.
C. It is normally assembled in the nucleus and exported to the cytoplasm by exportins.
D. It is an ER integral membrane protein that can interact with the translocon.
E. It requires high salt concentration for its function in vivo.

Answer 1

The 11S particle is an ER integral membrane protein that can interact with the translocon to restore the protein-translocation activity of salt-extracted microsomes.

Step-by-step explanation:

The correct answer is D. It is an ER integral membrane protein that can interact with the translocon.

The 11S particle is a protein complex that plays a crucial role in protein translocation. It is not composed of 21 proteins (option A) or a digestion product of ER-associated ribosomes (option B). It is also not assembled in the nucleus and exported to the cytoplasm by exportins (option C). The 11S particle is an ER integral membrane protein that interacts with the translocon to restore the protein-translocation activity of salt-extracted microsomes. It does not require a high salt concentration for its function in vivo (option E).