Protein Trafficking Flashcards
Where are cytosolic proteins made?
How?
In the cytosol
On free ribosomes through a ‘simple cycle’
Where are membrane and secretory proteins made?
How?
In the rough endoplasmic reticulum (RER)
Through an SRP cycle
Where are nuclear proteins made?
In the outer nuclear membrane
Where do the proteins made in the RER travel to? (in order)
- Smooth ER
- Golgi
- Secretory vesicles
- Membrane
Why is the RER ‘rough’?
Due to ribosomes
Where does ALL protein synthesis START?
In the cytosol
Describe the SRP cycle
ER signal sequence in the mRNA binds to the signal recognition particle and directs the ribosome to the ER membrane
The mRNA then remains permanently bound to the ER as part of a polyribosome (as the ribsomes translate the mRNA - the protein sequence falls into the ER lumen)
Ribosomes move along the mRNA and are recycled at each round of replication to join the free ribsome pool
Where do proteins travel to after they are synthesised in the RER?
To the edges of the ER where there are no ribosomes - the SER
Where are LIPIDS made?
In the SER
What happens to lipids and proteins in the SER?
They are packaged together in a vesicle - to travel towards the golgi and further on
What happens in the SER?
Lipids are made and packaged with proteins into vesicles
What is used to separate the heavy and light vesicles?
Describe this process
Differential centrifugation:
- Gradient of sucrose concentration
- Spin at high speed
- Low density settles at the bottom
- High density settles at the top
What are the high density vesicles?
Studded ribosomes - ER vesicles that studded with ribosomes
How are proteins sent to the right destination in the cell?
Based on their signal sequence
What are signal sequences?
The first 10-12 amino acids on the polypeptide chain
What type of signal sequence is needed to direct the protein to the nucleus?
What is this sequence different to?
Positive charges
These sequences are different to the signal sequences that destines the proteins to the ER
What does the SRP do?
Attaches to and guides free ribosomes to the ER by binging to the signal sequence on the growing poly peptide chain
What does the SRP consist of?
Cellular proteins
How does the SRP attach to ribosomes?
With its hinge region
What happens after the SRP is bound to the signal sequence?
- Pause in translation
- SRP receptor in the ER membrane binds the SRP-ribosome complex
- Directs the complex to the to a protein translocator in ER membrane
- Signal sequence is captured by the translocator - opening the channel
- Ribosome is passed to the translocator and translation continues
- SRP and receptor are released and recycled
What is the topography of the ribosome on the ER membrane?
Small ribosome subunit facing to cytosome
Large ribosome subunit is attached to the protein translocator in the ER membrane
What removes the signal sequence from proteins in the ER?
ER-based signal peptidases (specific signal peptidases)
What happens if the proteins being synthesised on the ER membrane have a stop-transfer sequence?
The protein becomes embedded in the ER membrane
What happens if the proteins being synthesised on the ER membrane don’t have a stop sequence?
What can this protein then be destined for?
ALL of the protein goes into the ER lumen
Destined for secretion or into the lysosomes