Lecture 11 - Translation, translocation and insertion

Question 1

What are targeting signals?

Answer 1

Targeting signals are the pieces of information that enable the cellular transport machinery to correctly position a protein. Information is contained in the polypeptide chain or in the folded protein. A continuous stretch of a.a. residues in the chain that enables targeting are called signal or targeting peptides

Question 2

What is SRP?

Answer 2

SRP (signal recognition peptide) is a cytosolic ribonucleoprotein of proteins bound to an RNA scaffold
SRP consists of 6 proteins and RNA which is 300bp - recognises polypeptide
SRP docks with SRP receptor on cytosolic face of ER
Interaction triggers GTP hydrolysis & release of SRP
-On SRP release, nascent polypeptide enters translocon membrane channel
-As it grows, nascent polypeptide is fed through translocon into ER
-Signal sequence is removed inside ER by signal peptidase enzyme
SRP causes translational arrest so the polypeptide does not continue to be made - this prevents the protein from folding up - if folded can’t go through translocon - and causes SRP to bind to its receptor. Both SRP and its receptor are GTPases - GTP hydrolysed to GDP and phosphate. This hydrolysis of the GTP allows the SRP to dissociate from the ribosome and go back to target more peptides but then also allows the ribosome with extended polypeptide to bind to the translocon which causes the translocon to have an open conformation. Signal peptidase cleaves off signal sequence as it is no longer needed. Protein continues to be made and if there is no further hydrophobic region you end up with a folded protein in the cytoplasm

Question 3

How do proteins enter the ER?

Answer 3

In cotranslational import, proteins to be targeted to the endoplasmic reticulum initially have an N-terminal peptide, the ER signal sequence, translated by a cytosolic ribosome.
The ER signal sequence is bound by a signal-recognition particle (SRP), a ribonucleoprotein complex composed of 6 peptides and a 300 nucleotide RNA molecule.
The SRP binds to the SRP receptor to dock the ribosome on the ER membrane.
When the SRP receptor binds GTP, the nascent polypeptide enters the pore.
The SRP is released with hydrolysis of the GTP.
The growing polypeptide translocates through a hydrophilic pore created by one or more membrane proteins called the translocon.
The most recent evidence suggests that the ribosome fits tightly across the cytoplasmic side of the pore and that the ER-lumen side is somehow closed off until the polypeptide is about 70 amino acids long.
When the polypepide is complete, the signal peptidase cleave the signal to release the protein into the ER lumen while retaining the signal peptide, for a time, in the membrane.
Afterwards the ribosome is released and the pore closes completely.

Question 4

What is the Sec61 complex?

Answer 4

Polypeptide chains enter the ER through a protein channel – the translocon
Cross-linking experiments with the translocon have shown where the signal sequence binds
Translocon can be purified and reconstituted to yield and active protein channel
Post-translational – requires the Sec61 complex and another larger Sec63(62,71+72) complex because the SRP and its receptor are not involved
Signal peptide binds to Sec61 as well as SRP

Question 5

How does post-translational insertion into the ER take place?

Answer 5

An additionalcomplexcomposed of the Sec62, Sec63, Sec71, and Sec72 proteins is attached to the Sec61 translocator and deposits BiP molecules onto the translocating chain as it emerges into theER lumen. ATP-driven cycles of BiP binding and release pull the protein into the lumen, a mechanism that closely resembles the thermal ratchet model for mitochondrial import
BiP is chaperone in the ER lumen that hydrolyses ATP and pulls polypeptide into the ER using that energy.