RAN system and Nuclear transport Flashcards

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1
Q

Describe the theory for formation of a eukaryote.

A
    • An archaea loses its cell wall.
    • This increases horizontal gene transfer, speeding evolution.
    • digestion of other prokaryotes also increases speed of evolution
    • membrane begins to invaginate to allow a more and more protected nucleus.
    • A cell is consumed that is not digested, it is the 1st mitochondria
    • Nucleus also helps to form the ER.
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2
Q

G Proteins

A

Bind GTP, hydrolyze to GDP. GTP is considered generally, the active state.

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3
Q

GAP

A

GTPase Accelerating Proteins. Catalyze GTP–>GDP

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4
Q

GEF

A

Guanine Nucleotide-Exchange Factors. Help with release of GDP from the G protein.

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5
Q

What are the two types of G-Proteins?

A

Heterotrimeric G proteins
Have three subunits, an Alpha (Which has GTPase activity), beta and gamma, form a heterodimer. Heterotrimeric G proteins often couple with G protein-coupled receptors (GPCRs).

Monomeric G proteins

  • Evolutionary speaking, related to the alpha protein of the heterotrimeric G proteins.
  • Exists in thousand varieties
  • Ras was the first monomeric G protein discovered.
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6
Q

Where is RAN-GEF located?

A

Within the Nucleus. So RAN-GTP will exist exclusively in the nucleus.

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7
Q

Where is RAN-GAP located?

A

Within the Cytoplasm. So RAN-GDP will exist exclusively in the cytoplasm.

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8
Q

Is RAN monomeric or trimeric?

A

Monomeric G protein.

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9
Q

What is a Nuclear Import Receptor?

What is a Cargo Protein?

A

A Nuclear Import Receptor is a protein which can bind to RAN-GTP or a Cargo Protein (mutually exclusive). It has poor binding affinity for RAN-GDP.

A cargo protein is any protein carried in or out by import or export receptors.

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10
Q

What is a Nuclear Export Receptor?

A

It is a protein which can only bind it’s cargo protein if RAN-GTP is bound to it. It has low affinity for RAN-GDP (and no affinity for the cargo protein once RAN-GTP leaves).

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11
Q

Explain Nuclear Import.

A

A Nuclear Import Receptor binds a cargo protein. It travels into the nucleus. Inside the nucleus is where RAN-GTP dwells. RAN-GTP binds to the nuclear import receptor, either after it has already released its cargo protein or making it release the cargo protein as it binds. With RAN-GTP bound, the Nuclear Import Receptor cannot bind its cargo protein. Once it leaves the nucleus, RAN-GAP jumps it, and catalyzes GTP–>GDP. GDP has poor affinity for its current binding site, and falls off, leaving the site open for a binding protein.

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12
Q

Explain Nuclear Export.

A

A Nuclear Export Protein must bind RAN-GTP to bind its cargo protein. RAN-GTP is in the nucleus, so we will start our tail with a Nuclear Export Protein who has bound both RAN-GTP and its cargo protein. Once it has bound its cargo protein, the Nuclear Export Protein skedaddles to the cytosol, where RAN-GAP takes RAN-GTP –> RAN-GDP in ~10 ms. Binding of cargo protein is now impossible, so it leaves. The Nuclear Export Protein is returned to the nucleus, ready to undertake its journey once again.

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13
Q

Even though the slides didn’t describe one, why must there be a mechanism to move RAN-GDP into the nucleus?

A

Because RAN-GEF exists only in the nucleus. RAN-GDP rarely releases GDP without RAN-GEF and there is little RAN-GTP in the cytosol (due to RAN-GAP).

Given this, we know the only way to get more RAN-GTP, is to move RAN-GDP to the nucleus, where RAN-GEF can release GDP and GTP can bind to RAN.

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14
Q

Is GTP or GDP kept in higher concentrations in the cell? How much higher.

A

GTP is kept ~20x higher. Therefor by statistics alone it will bind to RAN more often.

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15
Q

What are nuclear lamina?

A

They are an internal structure of the membrane that looks like burlap sacks, and binds to protein pores.

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16
Q

What is the lamina made of?

A

Intermediate fibers named lamins.

17
Q

Phosphorylation of the lamina does what? What phosphorylates it.

A

Causes the loss of stability and the breakdown of the nuclear envelope. This is phosphorylated by CDK (cyclin dependent kinase).

18
Q

What does dephosphorylation of lamins cause?

A

The reassemblage of the nuclear envelope.

19
Q

Is it possible to have exclusions of receptors to specific regions or from the whole nuclear envelope.

A

Yes, this goes down. It changes expression in specific areas.

20
Q

What is calcineurin?

A

Is a protein phosphatase which is only active when Ca2+ is present in high concentration in the cell.

21
Q

NFAT stands for what?

A

Nuclear Factor of Activated T-cells.

22
Q

What does NFAT do once inside the nucleus?

A

It is an activator of transcription.

23
Q

Explain how the NFAT regulatory system works.

A

NFAT starts in the cytosol. Where it sits, bound to three phosphates, which have shifted its conformation so it shows a cytosolic signal sequence. Increase in intracellular calcium activates calcineurin, a phosphatase, which removes the three phosphates on NFAT. This causes a nucleus signal sequence to be shown. NFAT enters the nucleus, and transcription goes down. Until Ca2+ lowers and a kinase adds 3 phosphates like nobodies business. NFAT has a signal sequence saying pack it all up and move to the cytosol. And NFAT just listens, because that’s the type of Nuclear Factor of Activated T-cell it is.