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M2MWeek6 > Import/Export of Nucleus > Flashcards

Import/Export of Nucleus Flashcards

1
Q

What types of molecules are imported into the nucleus?

A

RNA pols; DNA pols; Protein kinases/phosphates; Histones; RNA binding proteins (also exported); snRNA (also exported); transcription factors (also exported)

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2
Q

What types of molecules are exported from the nucleus?

A

40s and 60s ribosomal subunits; tRNAs; mRNAs; RNA binding proteins (also imported); snRNAs (also imported); transcription factors (also imported)

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3
Q

What is the structure of the translocation channel?

A

Macromolecular pores (nuclear pore complexes; NPCs)

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4
Q

What are the major structural features of the NPC?

A

~30 distinct “nucleoproins” (Nups) repetitively arranged.

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5
Q

What are the 3 layers of the NPC?

A

Nuclear envelope layer; scaffolding layer; barrier layer

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6
Q

What are the main features of each layer in the NPC?

A

Nuclear envelope layer = Lumenal ring. Scaffoling layer = Inner and outer rings; linker Nups; Cytoplasmic filaments; nuclear basket. All attached to membrane layer/lumenal ring. Barrier layer = FG Nups (form central channel)

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7
Q

What is a key feature of the Barrier Nups?

A

FG repeats (phenylalanine;glycine). Create hydrophobic patches. Highly disordered; but achieve order when they are affiliated with other components (key element of transport)

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8
Q

What types of molecules can freely traverse the nuclear envelope (diffusion driven)?

A

Small and hydrophilic

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9
Q

If a molecule is large how does it pass through the nuclear envelope?

A

Must have hydrophic patches (be an ampiphilic molecule). Rapidly associates/deassociates with FG repeats and gets transported through envelope. At some point it must reach a conformational change within the nucleus so it can disassociate from the FG repeats. This is spontaneous migration (needs no energy input)

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10
Q

What is the 3rd way a molecule can pass through the NPC?

A

Facilitated transport (requires a transporter). Requires energy input

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11
Q

What are the main principles of NPC facilitated transport?

A

There is a cargo molecule which is hydrophilic and has a signal region (EX NLS or NES) for the carrier. The carrier is usually amphiphilic. They bind and the cargo is transported across the envelope (and across its concentration barrier). Inside the nucleus there is an energy couple dissociation. Renders this process irreversible

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12
Q

What is a key feature of the signal regions (NLS and NES)? Why is this important?

A

Signals must be on the outside of the protein to be active because proteins remain folded during transport

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13
Q

What is the general term for the cargo transporter/carrier protein?

A

Karyopherins AKA importins/exportins

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14
Q

What are the two classes of karyopherins?

A

Receptor family (eg Karyopherin Beta) and Adapters (eg Karyopherin alpha)

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15
Q

How do receptor family karyopherins work?

A

Bind directy with cargo and interact directly with FG Nups

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16
Q

How do adapter family karyopherins work?

A

Bind directly with specific cargos but cannot themselves interact with FG Nups. Will bind with the cargo and another receptor family Karyopherin to form a heterodimeric import receptors. All 3 will pass through the envelope

17
Q

What is NTF2?

A

Specific transporter for Ran. GDP (Note: NTF2 is not of the karyopherin family)

18
Q

What is NXF1/NXT1

A

Transporters for mRNA and rRNA (rRNA requires this plus karyopherins. Note: NXF1 and NXT1 are not of the karyopherin family)

19
Q

What does Ran.GTP do?

A

Once the cargo/karyopherin are inside the nucleus; Ran.GTP binds to the karyopherin to cause conformational change so it will un-bind from the cargo. Ran.GTP-karyopherin complex is then transported back outside the nucleus where Ran.GTP is hydrolyzed to Ran.GDP and the two complexes break apart

20
Q

How much does transport cost with just a receptor family transport (beta subunit)? How much if there is adaptor + receptor (alpha + beta)?

A

1 GTP/molecule for just receptor. 2 GTP/molecule for alpha + beta

21
Q

How does facilitated export through the NPC occur?

A

Cargo + transporter + Ran.GTP all bind in nucleus then pass through NPC then break apart in the cytoplasm. NTF2 brings Ran.GDP back to nucleus. Transporters come back through simple diffusion

22
Q

What proteins are needed to convert Ran.GTP into Ran.GDP? Where does this occur?

A

RanBP1 most important. Also RanGAP and RanBP2 (tethered directly to cytoplasmic filaments on outside of nuclear envelope). Happens in cytoplasm

23
Q

What proteins are needed to convert Ran.GDP into Ran.GTP? Where does this occur?

A

RCC1 (tethered directly to chromatin). Happens inside nucleus

24
Q

Are the NLS/NES cleaved during transport?

A

No. So they can be re-used

25
Q

How is energy spent during transport?

A

Not directly couple to transport; but linked to the recylcing of Ran

26
Q

What 2 things can Importin Beta bind to?

A

Can bind to Importin Alpha (the adapter protein) or directly to the cargo

27
Q

Can proteins contain both NLS and NES?

A

Yes. Proteins/protein complexes can shuttle so many contain both.

28
Q

How abundant are the Ran proteins?

A

Very. About 0.4% of total cell protein

29
Q

What energy molecule drives transport of mRNA out of nucleus? What is it used for?

A

ATP. Required to remove certain proteins as mRNA passes through the NPC (and maybe attached cytoplasmic Rnbp?)

30
Q

When does NXF1/NXT1 transporters attach to mRNA

A

In nucleus after splicing; etc. After removal of nuclear Rnbp

31
Q

What are 3 ways regulation of nucleocytoplasmic transport can occur?

A

Through regulation of NPC (permeability/protein expression); transport receptors (availability of); or cargo (Post-translational or transcriptional modifications; inter/intramolecular interactions)

32
Q

What is the main way regulation occurs?

A

Through regulation of cargo

33
Q

How is disruption of nucleo-ctyoplasmic transport seen in cancer?

A

Normally BRCA2 and RAD51 are sequestered inside the nucleus; in cancer cell they are both exported from the nucleus. In some cancers Crm1 protein is over expressed and so p53 gets exported from nucleus

34
Q

How does SINE (selective inhibitors of nuclear export) work in cancer therapy?

A

Binds to Crm1 protein to prevent export

M2MWeek6 (8 decks)

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