Question

1. Which cytosolic factor is most likely to associate with the signal sequence of a protein that is targeted co-translationally to the ER?

A chaperone of the Hsp70 family

The translocation channel

The ribosome

Signal recognition particle (SRP)

Signal peptidase

2. What is the most likely fate of a protein with an N-terminal hydrophobic sorting signal and an additional internal hydrophobic domain of 22 amino acids?

The protein stays in the cytosol

The protein is transported to mitochondria

Because the protein has an N-terminal sorting signal, the protein is translocated all the way into the ER lumen

The hydrophobic domain is recognized as a transmembrane domain once it is in the translocation channel and released sideways into the membrane

The hydrophobic domain is cleaved off by signal peptidase

3. Which of the following statements best describes N-glycosylation? N-glycosylation refers to........

the addition of a pre-formed sugar tree to a serine or threonine present in the cytosolic domain of a protein

the addition of a pre-formed sugar tree to a phospholipid to make a glycolipid

the addition of individual sugars to selected amino acids on a protein

the addition of a pre-formed sugar tree to an asparagine residue found within the consensus sequence Asn-X-Ser/Thr in an ER lumenal domain of a protein

the addition of a pre-formed sugar tree to an asparagine residue found within the consensus sequence Asn-X-Ser/Thr in a cytosolic domain of a protein

4. What is the most likely fate of a protein in which a mutation has led to misfolding in the ER?

It is sequestered by the proteasome in the cytosol

It is first tagged with ubiquitin in the ER and then degraded by the proteasome in the cytosol

It is first moved out of the ER, tagged with ubiquitin in the cytosol and then degraded by the proteasome in the cytosol

It associates with many chaperones in the ER, which guarantee its correct folding

Its misfolded state is recognized by chaperones in the ER, which will induce the protein's degradation by the proteasome in the ER.

Answer 1

• Answer 1: SRP (signal recognition particle).
• Proteins destined for targetting into ER contain a SRP which is recognised by SRP receptors on ER membrane and the protein is internalized.

Ans2: The hydrophobic domain is recognized as a transmembrane domain once it is in the translocation channel and released sideways into the membrane. (D)

Ans 3: the addition of a pre-formed sugar tree to an asparagine residue found within the consensus sequence Asn-X-Ser/Thr in an ER lumenal domain of a protein.(c).

Ans 4: Its misfolded state is recognized by chaperones in the ER, which will induce the protein's degradation by the proteasome in the ER. (D)