L5- The ER Flashcards
(39 cards)
What is the ER’s structure?
Continuous with outer membrane, forming a dynamic system of interconnected tubules and sacs.
What is rough ER?
When bound ribosomes are present.
How much ER is there in a cell?
Amount of ER= cell type specific. e.g. in the pancreas lots of ER because it is secretary
What are the ERs functions?
- Protein synthesis
- Entry point into the secretary pathway
- Supplies newly made proteins and lipids to the compartments of the secretary pathway and the plasma membrane. (golgi, lysosome, and pm)
What is the schematic secretary pathway?
ER–> Vesicle–> Golgi –> vesicle–> PM
What happens to ribosomes that synthesis proteins without a signal sequence?
They remain free in the cytosol
What happens to ribosomes that do make an ER signal sequence?
They’re directed to the ER membrane while the protein is still being made. The proteins are then translocated across the membrane into the ER.
What is co-translational translocation?
This process of proteins being threaded through the ER membrane while they are being synthesised by the ribosome. (v. different from post translational process like in nucleus)
What is SRP?
Signal recognition particle (6 proteins and an RNA backbone)
What does SRP do?
Binds to ER targeting signals as they emerge from the ribosome. It has a sting of hydrophobis residues. SRP receptor in the ER membrane recognises and binds the SRP-polypeptide-ribosome complex
What do SRP and the SRP receptor contain?
GTP binding proteins
What is the SRP dependent targeting cycle to the ER regulated by?
GTP binding and hydrolysis
What happens when the ribosome/SRP complex binds to the SRP receptor at the ER membrane?
The SRP is released.
The ribosome is transferred to the protein translocator and the polypeptide is threaded through this channel as it’s being made.
(this translocation channel opens when it binds the ER signal sequence)
What is the protein translocator in the ER membrane?
The sec61 complex
What connects ribosomes with ER signalling proteins to available translocation channels in the ER membrane?
SRP and the SRP receptor
What happens after targeting for the soluble proteins that will go right into the ER?
The signal sequence will remain bound to keep the translocon open. The rest of the protein is threaded through as a large loop into the ER lumen. (translocation)
The ER signal sequence is then cleaved off in the lumenal side by a signal peptidase. (so whole protein is in the ER)
What happens after targeting for the simple membrane proteins?
Integral membrane proteins have an ER targeting signal and a stop-transfer signal.
The stop-transfer signal is released sideways from the ER translocon, allowing the protein to become embedded in the membrane. = integration.
N-terminal signal sequences are cleaved.
Signal-anchor sequence acts as both a targeting and stop-transfer signal and is not cleaved.
What happens after targeting for complex membrane proteins with multiple transmembrane segments?
Made up of combinations of hydrophobic start and stop transfer signals. Also referred to as signal-anchor sequences. All of these signal sequences are inserted into the ER membrane via the sec61 complex.
Why are tail-anchored proteins not recognised by SRP?
Their hydrophobic ER targeting signal is at the C-terminus so as they emerge from the ribosome they’re not recognised by SRP.
What does the ER targeting signal in tail-anchored proteins act as?
A membrane anchor
How are tail-anchored proteins inserted into the ER membrane?
Post-translational insertion (as SRP doesn’t attach)
What’s an important example of tail-anchored proteins?
SNARE proteins
How are proteins modified at the ER?
- Cleavage of signal sequences
- Disulphide bond formation
- Glycosylation (addition of carbohydrate)
How are disulphide bonds formed?
Oxidation of cysteine side chains-stabilise folded structure.
Catalyzed by enzyme in ER lumen -> protein disulphide isomerase =PDI
