MCB Lecture 31 Gated Transport & Transmembrane Transport Flashcards Preview

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Flashcards in MCB Lecture 31 Gated Transport & Transmembrane Transport Deck (35)

How do molecules get in and out of the nucleus?

Gated transport


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?

Lysine rich
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?

Signal sequence


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?

Soluble proteins


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 doesn't
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


How does the signal sequence of proteins bound for the ER associate with the ER?

Signal recognition particle (SRP)

SRP receptor protein

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