Flashcards in MCB Lecture 31 Gated Transport & Transmembrane Transport Deck (35)
How do molecules get in and out of the nucleus?
Describe the structure of a nuclear pore
Octamer: eight proteins line the pore complex
How do larger molecules get across the nuclear envelope?
Need a translocator
1. Nuclear Localisation Sequence found in protein
2. Nuclear Import Receptor recognises the NLS
3. Protein is shuttled through the Nuclear Pore Complex on the Nuclear Import Receptor
4. Ran-GTPase hydrolysis GTP, cargo dissociates in the nucleus
5. GDP dissociates on the cytosol side
What is the name of the sequence that is recognised on a protein for its import into the nucleus?
Nuclear localisation sequence
What is the name of the receptor that moves proteins into the nucleus?
Nuclear import receptor
What are the features of the nuclear localisation sequence?
15-60 amino acids long
Describe the steps involved with exporting a protein from the nucleus
Same as the import, except the protein binds to the Nuclear Import Receptor on the inside, as well as Ran-GTP.
Ran-GTP is hydrolysed in the cytosol, and the cargo is released
Describe the function of Ran-GTPase
Hydrolysis GTP in the cytosol
Where are nuclear proteins synthesised?
In the cytosol
When do nuclear proteins fold?
In the cytosol
How is the import of nuclear proteins regulated?
Phosphorylation of a protein causes conformational change, and the Nuclear Localisation Sequence is exposed
Now, the protein can be imported
When is transmembrane transport used?
To move protein from the cytosol into the mitochondria or ER
Describe how proteins traverse the two mitochondrial membranes
1. Signal sequence on protein recognised by the TOM receptor
2. TOM and TIM line up in the membranes -> translocation channel
3. The protein is fed through into the matrix of the mitochondria
4. Signal sequence cleaved
5. Protein folds
Describe the features of the signal sequence for proteins entering the mitochondria
The signal sequence is an amphiphilic sequence.
The receptor I recognises the hydrophobic region
Describe the structure of the translocation channels on the mitochondrial membrane
TOM on outer membrane
TIM on inner membrane
How do proteins get into the peroxisomes?
Translocators. The mechanism is not well understood
What is cotranslational translocation?
This is when a protein is fed into the ER while it is still being translated
What is the sequence that is recognised on a protein so that it many enter the ER?
What is the signal recognition particle?
This is a separate protein that binds to the signal sequence on a protein that is being fed into the ER
When is binds, it is then recognised by the Signal Recognition Particle Receptor
The protein is fed through the translocator
What is the Signal Recognition Particle Receptor?
This recognises the Signal Recognition Particle bound to the signal sequence of a protein being translated
What is Sec61?
This is the translocator complex and the ribosome in the ER membrane
Which types of proteins are fed through an ER translocator?
What is signal peptidase?
Cleaves the signal sequence, so that the protein is free in the lumen of the ER
Where do ER proteins fold?
In the lumen of the ER
What are the two binding sites on the Nuclear import receptor?
What is special about the binding sites?
1. Cargo binding domain
2. Ran binding domain
The nuclear import receptor can only bind one of these at a time
What is Ran?
It is a protein that binds GTP and hydrolysis it.
It is important in the movement on proteins in and out of the nucleus
What sort of molecules are shuttled across the nuclear pore by nuclear import and export receptors?
What is the name of the receptor that exports proteins from the nucleus across the nuclear pore complex?
Nuclear export receptor?
How does binding of RAN-GTP differ in nuclear export and import?
It binds in the nucleus, moves across and hydrolysed and dissociates in the cytosol
What is interesting about the NFAT protein that is shuttle across the nuclear pore?
The NLS is cryptic, but is revealed when phosphorylated by a Ca2+ kinase
Thus nuclear import is regulated and calcium dependent
How is it ensured that mitochondrial proteins do not fold before they enter the mitochondria?
Chaperones bind to them
What is the structure of the signal sequence of a protein that is recognised by the translocator receptor?
It is an alpha helix
Which proteins enter the ER?
Proteins destined for membrane embedding
Proteins bound for lysosomes, Golgi and ER
Describe the locations involved for translation of proteins that are bound for the ER
Initiation: ALWAYS in the cytosol on free ribosomes
Termination: ribosomes bound to the endoplasmic reticulum