Protein Targeting Flashcards
What does SRP stand for?
Signal recognition particle
When proteins are directed to the ER is their signal sequence cleaved or not? If so, when
Yes, the hydrophobic signal sequence is cleaved by signal peptidase upon entering the ER lumen
At which protein terminus is the signal sequence when directed to the ER?
N-terminus
When proteins are directed to the ER, is energy needed? If so how and when?
What is its consequence?
Yes- GTP is converted to GDP which releases energy to cause the SRP to dissociate from the SRP receptor on the ER membrane
Allows ribosome to move along over the transport protein and translation continues directly into ER lumen
When proteins are directed to the ER, is the protein folded or unfolded?
Unfolded, it folds once ribosome is released
What sequence do proteins have that are meant to remain in the ER?
KDEL sequence
What and where is the KDEL sequence recognised?
By KDEL receptors in the Golgi
They are able to bind to the sequence due to lower pH
What other organelle is involved in ensuring proteins destined for ER remain there?
Golgi- when they are accidentally secreted out of ER
When protein are being directed to the ER, what sort of signal sequence is recognised? And what is it recognised by?
Hydrophobic signal sequence
SRP- signal recognition particle
What face of the Golgi takes in secretory vesicles containing proteins from the ER?
Cis face
Describe points in returning a protein meant for ER to the ER.
- energy from GTP allows formation of vesicle with protein in
- vesicle taken into Golgi
- KDEL sequence binds to KDEL receptor due to low pH
- vesicle with protein bound to receptor buds off (needs GTP)
- goes to ER and taken into ER
- pH higher TF released into ER lumen
What is GTP?
Form of energy
Guanosine tri phosphate
When proteins directed to the ER are mistakenly released, is energy needed? In what form? And why?
Yes
GTP
To form secretory vesicle
When proteins are directed to the nucleus, do they enter nucleus folded or unfolded?
Folded already
When proteins are directed to the NUCLEUS, what signal sequence do they have and what binds to this?
Basic nuclear localisation sequence
Importin
What is ‘Importin’? What role does it play in protein transport?
Transport/carrier protein for folded proteins bound for the nucleus
When proteins are directed to the NUCLEUS, is energy needed? In what form? And why?
Yes
Ran-GTP
Binds to Importin to release protein, hydrolysed to Ran-GDP to release Importin
When proteins are directed to the NUCLEUS, is the signal sequence cleaved or retained?
Retained
What do proteins enter into the nucleus through? And bound to what?
Nuclear pore bound to Importin
When proteins are targeted for a LYSOSOME, are they folded or unfolded?
Folded
When proteins are targeted for a LYSOSOME, what other organelle plays a role?
Golgi body
What form of post-translational modification occurs to proteins that are targeted for a LYSOSOME?
Addition of a mannose-6-phosphate
Mannose is a sugar
When proteins are targeted for a LYSOSOME, is energy required? If so what form and what for?
Yes
ATP
Pumping of H+ ions into late endosome to create acidic environment
In the late endosome, why are H+ ions pumped in?
To lower the pH inside
This allows the receptor molecule bound to mannose-6-phosphate to be released
When proteins are targeted for a LYSOSOME, is the receptor molecule recycled?
Yes, the M6P receptor is released and sent back to the Golgi body in a transport vesicle
When proteins are targeted for a LYSOSOME, is the signal sequence cleaved? What is the signal sequence?
Yes- removed by phosphatase enzyme
Mannose-6-phosphate which is added on in Golgi body
When proteins are targeted for a MITOCHONDRIA, is the protein folded or unfolded?
Unfolded
It is kept unfolded by chaperones
Chaperones are added using energy from ATP hydrolysis
Where is the signal sequence on proteins destined for MITOCHONDRIA? What is it?
Amphipathic sequence on the N terminus
Define amphipathic
Both hydrophobic and hydrophilic
Describe how a protein would enter the mitochondria.
- Amphipathic Signal sequence binds to receptor on outer membrane
- this allows protein to be fed through TOM pore in outer membrane
- as inner and outer so close- go through both at same time
- goes through TIM pore in inner (both these require ATP)
- signal sequence cleaved, triggers chaperones to release
- protein can fold
