Lecture 26 - Nucleo-Cytoplasmic Transport

Question 1

What is added to the end of an RNA made in the nucleus?

Answer 1

Poly-A Tail

Question 2

Through what do molecules transit between the nucleus and the cytoplasm?

Answer 2

Nuclear Pore

Question 3

How do the cytoplasmic face and nucleoplasmic face of the nuclear pore differ?

Answer 3

The cytoplasmic face has a classic structure that is easy to identify. The nuclearplasmic face looks very strange and has a basket structure.

Question 4

What proteins are the nuclear pore complex made of?

Answer 4

Nucleoporins

Question 5

What does transport through the nuclear pore complex depend on?

Answer 5

Reversible Hydrophobic Interactions

Question 6

What class of nuclearporins mediate reversible hydrophobic interactions?

Answer 6

FG Nucleoporins

Question 7

How do FG repeats affect transport through the NPC?

Answer 7

The FG repeats form a gel-like matrix that is very hydrophobic. They together form the hydrophobic moments that, in this gel kind of situation, don’t usually let the free flow of bigger molecules through, particularly proteins and some nucleic acids.

Question 8

What does a proteins interaction with T-transporters allow for? How?

Answer 8

T-transporters permit flow through the NPC for proteins it is interacted with. The transporters mediate the interaction with the FG repeats and in doing so allow the movement of key molecules through the pore.

Question 9

What does the common motif of nuclear-bound proteins consist of?

Answer 9

The motif is usually made up of basic amino acids, like lysine and arginine.

Question 10

What do nuclear localization signals/sequences (NLS) do?

Answer 10

They interact with nuclear transporter proteins so that proteins can be brought through the nuclear pore and eventually into the nucleus.

Question 11

What organism was studied to better understand NLS?

Answer 11

SV40 (a Virus)

Question 12

What can be said about some mutations of the T-antigen made by SV40? What did the mutations that didn’t make it show?

Answer 12

Some mutated T-antigens never made it into the nucleus. The mutations were used to identify the NLS, specifically that it consists of just a few amino acids.

Question 13

What would happen if you were to introduce a NLS into a cytoplasmic protein?

Answer 13

It would suddenly become nuclear-localized.

Question 14

What G-protein is important for nuclear import?

Answer 14

Ran

Question 15

What are importins? How do they function?

Answer 15

Importins are important proteins for taking cargo from the cytoplasm into the nucleus. They do this through their ability to interact with FG repeats within the nuclear pore and the nucleoporins that form the gel.

Question 16

What is the NLS recognized by?

Answer 16

Importins

Question 17

What causes the importin to release its bound protein? What does the importin do after this happens?

Answer 17

When it enters the nucleus, the complex is met by Ran in its GTP-bound form. This causes a conformational change that allows the importin to let go of its bound protein. Then, the importin and Ran GTP will make their way out of the nucleus and head for the cytoplasm.

Question 18

Where will Ran GAP hydrolyze Ran GTP?

Answer 18

Cytoplasm

Question 19

What happens to Ran GDP once it is formed from Ran GTP?

Answer 19

It moves into the nucleus.

Question 20

What drives the nucleo-cytoplasmic movement of Ran GTP/GDP.

Answer 20

Concentration Gradient

Question 21

Where is Ran GEF found?

Answer 21

In the Nucleus

Question 22

What proteins do mobile components of nuclear export interact with?

Answer 22

Exportins

Question 23

What is exportin-cargo protein interaction dependent on?

Answer 23

Ran GTP

Question 24

Where is Ran GTP concentration high?

Answer 24

Nucleus

Question 25

What is the tripartite complex in nuclear export composed of?

Answer 25

It consists of exportin, its bound cargo, and Ran GTP.

Question 26

What does the tripartite complex interact with to make it waythrough the NPC in nuclear export?

Answer 26

FG Nucleoporins

Question 27

Which protein is critical for exporting tRNA from the nucleus?

Answer 27

Exportin T

Question 28

Which type of RNAs typically don’t require exportins for nuclear export?

Answer 28

mRNAs

Question 29

What proteins make up the mRNA exporter?

Answer 29

Nuclear Export Factor 1 (NXF1) and Nuclear Export Transporter 1 (NXT1)

Question 30

What protein around the exons do the mRNA exporter proteins interact with to form a complex that can pass through the nuclear pore?

Answer 30

SR Proteins

Question 31

What are mRNPs?

Answer 31

Messenger Ribonucleoproteins; mRNA with bound proteins.

Question 32

Describe the simple model of mRNP export.

Answer 32

The mRNA exporter is recruited to a mature mRNA to be exported by its interactions with the SR proteins. Together they will make their way to the nuclear pore complex and interact with the FG repeats in order to go to the cytoplasmic side of the nuclear membrane.

Question 33

What is the RNA helicase on the cytoplasmic side of the NPC responsible for?

Answer 33

It strips the mRNA of all its nuclear proteins.

Question 34

What happens to the mRNA exporter when it is stripped from the mRNA?

Answer 34

It moves along its concentration gradient back into the nucleus.

Question 35

When mRNPs come to the nuclear pore, they interact with the basket. What does this do?

Answer 35

The interaction allows something to make their way through the pore, but not everything. mRNA makes it through the pore, but a lot of proteins don’t.

Question 36

What almost instantly interacts with mRNA once it reaches the cytoplasmic side of the nuclear membrane? What does this tell us?

Answer 36

Ribosomes

This tells us that mRNA must be passing the the NPC in a polarized manner where the 5’ end comes out first (since ribosome read mRNA 5’ to 3’).

Question 37

When are splicing factors removed from mRNA?

Answer 37

They are removed before the mRNA passes through the NPC.

Question 38

Where are the mRNA exporter and PABPN1 removed from the mRNA?

Answer 38

Cytosol

Question 39

What happens the moment that naked RNA becomes exposed in the cytoplasm?

Answer 39

When this happens, new cytoplasmic proteins jump on and take care of all the functions that are being addressed in the nucleus.

Question 40

When are pre-mRNAs exported from the nucleus?

Answer 40

They are exported after they have undergone splicing and become mRNAs.