Protein Sorting in Cell-organelles

Question 1

What are signal sequences? And how does it differ from and signal patch?

Answer 1

A signal sequence is a protein region with which a protein can be directed to the appropriate cellular compartment within a cell; they initiate co-translational transfer through the membrane of the endoplasmic reticulum (ER)

Signal sequences are removed by signal peptidases, whereas signal patches are retained.

Question 2

Which element recognises ER signal sequences?

Answer 2

Signal recognition particles (SRP), which in turn is bound by an SRP rector on the rough ER membrane.

Question 3

What is SRP comprised of?

Answer 3

1. One 300-nucleotide RNA.
2. 6 proteins (P9, P19, P54, P68, P72).
3. P54 binds to hydrophobic core of the protein signal sequence.
4. SRP receptor and P54 and GTPases.

Question 4

Which are the basic steps of ER translocation?

Answer 4

The SRP directs the binding of the ribosome to the ER and bind to the alpha subunit if the SRP receptor. When GTP is attached to the complex, the translocon channel is closed. When GTP is hydrolysed into GDP the translocon opens and the protein is inserted into the transmembrane channel. Signal peptidase cleaves the signal sequence.

Question 5

What is the role of BiP?

Answer 5

BiP is a molecular chaperone located in the lumen of the endoplasmic reticulum (ER) that binds newly synthesized proteins as they are translocated into the ER, and maintains them in a state competent for subsequent folding and oligomerization. BiP is also an essential component of the translocation machinery and plays a role in retrograde transport across the ER membrane of aberrant proteins destined for degradation by the proteasome.