The Signal Recognition Protein (SRP) Binds the Ribosome mRNA Polypeptide Complex to the ER membrane

Question 1

what would happen if translation were to continue without attachment to the ER?
what prevents this? (contrary to the signal sequence hypothesis)
what does this do?

Answer 1

• the folding of the growing polypeptide chain might bury the signal sequence
• contrary to the signal sequence hypothesis, the ER signal sequence itself does not initiate contact with the ER.
• Instead, the contact is mediated by a signal recognition particle (SRP)
• which recognizes and binds to the ER signal sequence of the newly forming polypeptide and then binds to the ER membrane

Question 2

SRP consists of both proteins and an RNA: (3)

Answer 2

1. one that binds to the ER signal sequence
2. one that interacts with the ribosome to block further translation
3. one that binds to the ER membrane

Question 3

Cotranslational import: (step 1)

• polypeptide synthesis proceeds until…
• at this stage, SRP binds to…?
• the SRP then binds the…?
• what is a tranlocon?
• what does a translocon contain? (2) for what reasons?

Answer 3

• polypeptide synthesis proceeds until the ER signal sequence has been formed and emerges from the surface of the ribosome
• at this stage, SRP binds to the signal sequence and blocks further translation
• the SRP then binds the ribosome to a special structure in the ER membrane called a translocon protein
• –>translocon is a protein complex than includes an SRP receptor and a ribosome receptor
• –> has pore protein that forms a channel to allow the polypeptide to enter the ER
• –>has signal peptidase to remove the ER signal sequence

Question 4

Cotranslational import: when does this process begin?

Answer 4

-process begins when an mRNA coding for a polypeptide destined for the ER starts to be translated on a free ribosome

Question 5

Cotranslational import: (step 2)

Answer 5

• SRP, with an attached ribosome, binds to the SRP receptor allowing the ribosome to bind to the ribosome receptor

Question 6

Cotranslational import: (step 3)

Answer 6

GTP binds both SRP and the SRP receptor, unblocking translation and causing transfer of the signal sequence to the pore protein, whose central channels open as the signal sequence is inserted

Question 7

Cotranslational import: (step 4)

Answer 7

GTP is then hydorlyzed, accompanied by release of the SRP

Question 8

Cotranslational import: (step 5)

Answer 8

as the polypeptide elongates, it passes into the ER lumen and signal peptidase cleaves the signal sequence, which is quickly degrded

Question 9

Cotranslational import: (step 6)

Answer 9

after polypeptide synthesis is completed, the final polypeptide is released into the ER lumen, the translocon channel is closed, and the ribosome detaches from the ER membrane and dissociates into its subunits releasing the mRNA