Nuclear transport

Question 1

What is the function of the nuclear pore complex (NPC)?

Answer 1

Bidirectional selective transportation of small polar molecules and large macromolecules.

Question 2

What is transported across the nuclear envelope by NCP?

Answer 2

1. Proteins: histones, DNA polymerases, RNA polymerases, transcription factors, RNA-processing proteins, ribosomal proteins, lamins…
2. RNAs: mRNAs, rRNAs, tRNAs…

Question 3

What is the nuclear pore complex (NPC) comprised of?

Answer 3

A set of 30 different nucleoporins, each nucleoporin occur multiple times, resulting in 500-1000 proteins in fully assembled nuclear pore complex.

Question 4

Can small water soluble molecules passively through nuclear pore complex?

Answer 4

Yes. Nuclear pore complex contain aqueous passage.

Question 5

How does the nuclear pore complex stop large macromolecules from freely diffusing in?

Answer 5

The channel nucleoporins with extensive unstructured regions forms a disordered tangle (FG meshwork core), it restricts the diffusion of big macromolecules while allowing small molecules to pass.

Question 6

What are FG repeats?

Answer 6

phenylalanine-glycine repeats in the unstructured domains of the channel nucleoporins that line the central pore.

Question 7

What are the functions of FG repeats

Answer 7

1. Interact weakly, form protein tangle gel-like properties that impose permeability barrier to macromolecules
2. Act as docking sites for nuclear import receptors

Question 8

How does the properties of phenylalanine and glycine contribute to the function of FG repeats

Answer 8

glycine have no side chain, so confers flexibility to the FG meshwork. NCP needs flexibility to accomodate different cargos

Question 9

Why is the structure of NPC flexible and dynamic?

Answer 9

To accommodate its diverse cargo.

Question 10

Describe the structure of the NPC

Answer 10

Question 11

What is the function of the nuclear localisation signal (NLS)

Answer 11

It is a sorting signal that directs nuclear proteins through the NPC and into the nucleus.

Question 12

What are the characteristics of the NLS

Answer 12

• Present on nuclear proteins and proteins that transiently enter the nucleus - highly conserved amino acid sequence - precise sequence varying for different proteins - Most contain lots of basic/positively charged amino acids lysine and arginine - Forms patches or loops on protein surface

Question 13

Where is the NLS located?

Answer 13

Can be located almost anywhere in amino acid sequence. Precise location mostly does not affect function. If one subunit of a multicomponent complex contains NLS then the whole complex gets imported into the nucleus

Question 14

What is karyopherins?

Answer 14

Karyopherins are nuclear transport receptors (importins + exportins).

Question 15

What are nuclear import receptors/importins?

Answer 15

Proteins that bind to both NLS and NPC proteins. Sometimes they bind to NLS through adaptor proteins.

Question 16

What is the function of nuclear import receptors?

Answer 16

Bind to NLS and guide cargo through the NCP

Question 17

Describe the model of how nuclear import receptors guide cargo through NPC.

Answer 17

• Receptor-cargo complex repeatedly bind, dissociate, and then re-bind to adjacent FG-repeat sequences
• When import receptors bind to FG repeats they dissolve the gel phase protein tangle and thus allowng cargo to pass
• Once inside the nucleus, import receptors dissociate with cargo and go back to cytosol.

Question 18

How is the directionality of the import process ensured?

Answer 18

Import receptors only dissociate with cargo in the nucleus. So cargo cannot go back out.

Question 19

How do macromolecules gets exported out of the nucleus?

Answer 19

• cargo have nuclear export signal
• Nuclear export signal binds to nuclear export receptors/exportins
• nuclear export receptors guide cargo through NPC to cytosol

Question 20

How do nuclear export proteins guide cargo through NPC?

Answer 20

The same way as nuclear import receptors do but in opposite direction.

Question 21

What is Ran?

Answer 21

• GTPase
• two states: GTP bound or GDP bound

Question 22

What is RanGAP

Answer 22

• Ran GTPase activating protein (GAP) –> triggers GTP hydrolysis –> turns RanGTP to RanGDP
• cytosolic

Question 23

What is RanGEF?

Answer 23

• Guanine exchange factor –> turn RanGDP to RanGTP
• nuclear
• anchored to chromatin

Question 24

What are helper proteins?

Answer 24

Proteins the help facilitate nuclear transport, they include e.g. CAS, NUP50, NTF2

Question 25

What are the concentration gradients of RanGTP and RanGDP in the cytosol and nucleus?

Answer 25

• RanGTP: high concentration in nucleus, low concentration in cytosol
• RanGDP: high concentration in cytosol, low concentration in nucleus

Question 26

How does the nuclear localisation of RanGTP ensures the directionality of nuclear import?

Answer 26

RanGTP binds to importin-cargo complex and unloads importins’ cargo. Because RanGTPs are solely in the nucleus, importins can only unload its cargo in the nucleus (in the cytosol, RanGDP does not bind to importins).

Question 27

Describe the Ran cycle.

Answer 27

1. Cargo binds importin
2. importin shuttles cargo through NPC
3. RanGTP binds to beta importin. Beta importin, alpha importin and cargo complex are dissociated as a result
4. alpha importin is taken up by CAS nuclear export factor, this process is catalysed by NUP50. Alpha importin in then exported out of nucleus by CAS
5. RanGTP, which is bound to beta importin is also exported out of nucleus
6. When beta importin-RanGDP complex arrives in the cytoplasm, RanGAP turns RanGTP to RanGDP, RanGDP and beta importin dissociates.
7. When alpha importin-CAS complex arrives in the cytoplasm, RanGAP turns RanGTP that is bound to CAS to RanGDP. So alpha importin, RanGDP and CAS all dissociate from each other
8. alpha importin and beta importin rejoin, ready to bind to new cargo
9. RanGDP taken up by NTF2 (nuclear transport factor 2), NTF2 shuttles RanGDP into nucleus
10. When RanGDP is in nucleus, RanGEF exchanges its GDP for GTP, thus turning it into RanGT

Question 28

How does nuclear export occur?

Answer 28

• RanGTP binds to exportin and promotes cargo binding to the exportin (instead of cargo unloading in nuclear import)
• Exportin shuttles cargo out through NPC
• In the cytoplasm, RanGTP bound to exportin is turned into RanGDP and so dissociates from exportin
• Free exportin returns to the nucleus to pick up new cargo

Question 29

Why can’t importin bind to both cargo and RanGTP at the same time?

Answer 29

RanGTP alters the conformation of the loop on the importin required for cargo binding