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MCB 2050 second half > the nucleus > Flashcards

the nucleus Flashcards

1
Q

2 main functions of the nucleus

A
  1. compartmentalization of the cellular genome and its activities
    - DNA replication, transcription, RNA processing
  2. coordination of cellular events
    - control of metabolism, protein synthesis, reproduction
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2
Q

what components make up the nuclear envelope

A
  • nuclear membrane
  • nuclear lamina
  • nuclear pores
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3
Q

what does nuclear content consist of (inside)

A
  • chromatin
  • nucleoplasm
  • nucleolus
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4
Q

nuclear envelope: structure

A
  • 2 parallel phospholipid bilayers
  • outer membrane binds ribosomes and is continuous with ER
  • inner membrane has integral membrane proteins that connect it to the nuclear lamina
  • inter membrane space is continuous with the ER lumen
  • inner and outer membranes join at nuclear pores
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5
Q

nuclear envelope: functions

A
  • separates nuclear content from the cytoplasm
  • selective barrier: passage of molecules from nucleus to cytoplasm, regulates gene expression
  • binds nuclear lamina (structural framework)
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6
Q

What is the nuclear lamina

A
  • thin meshwork bound to the inner membrane of the nuclear envelope
  • includes A, B and C lamins
  • supports structure of the nuclear envelope
  • scaffold for chromatin attachment
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7
Q

nuclear content: chromosomes

A
  • location of a gene is related to its activity (active genes found at periphery of a chromosomal subdomain)
  • inter-chromosomal channels are regions between domains as barriers between DNA-DNA or DNA-protein interactions
  • active genes (chromatin) from different subdomains extend to form transcription factories
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8
Q

Nuclear speckles

A
  • subdomains where mRNA splicing factories are concentrated
  • often located next to transcription factories
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9
Q

the nucleolus

A
  • NOT membrane-bound
  • site of ribosome biogenesis
  • initial stages or ribosomal subunit assembly (rRNA + protein)
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10
Q

nuclear pores

A
  • gateways between the cytoplasm and nucleoplasm
  • inner and outer membranes of the nuclear envelope fuse at pores
  • not evenly distributed across the membrane
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11
Q

Nuclear pore complex

A

protein structure that fills the nuclear pore
- fits into the pore and reduces the functional diameter
- extends to the cytoplasm and nucleoplasm

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12
Q

NPC structure

A
  • octagonal symmetry
  • structural and membrane nucleoporins
  • FG domains
  • cytoplasmic filaments
  • nuclear basket
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13
Q

structural and membrane nucleoporins

A
  • anchors the NPC to the nuclear envelope
  • forms an aqueous central channel
  • inner surface of the channel is lined by FG NUPS
  • located on the cytoplasmic and nuclear side of the NPC
  • link the central scaffold and cytoplasmic filaments or nuclear basket
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14
Q

FG Nups

A
  • FG nucleoporins have large FG amino acid repeats
  • FG domains possess highly disordered secondary protein structure
  • FG domains extend into the central channel
  • form a hydrophobic mesh which limits diffusion of macromolecules
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15
Q

cytoplasmic filaments

A
  • extend into the cytosol on the cytosolic side of the NPC
  • involved in nuclear receptor-cargo protein recognition and import
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16
Q

nuclear basket

A
  • located on the nuclear side of the NPC
  • involved in nuclear receptor-cargo protein import and export
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17
Q

functions of the NPC

A
  • passive diffusion of small molecules across the NPC
  • regulates the movement of larger molecules
    1. import of nuclear proteins and RNAs into the nucleus
    2. export of RNAs, ribosomal subunits and proteins from the nucleus
18
Q

what is nucleoplasmin

A

nuclear protein
- synthesized in the cytoplasm, associates with cytoplasmic filaments and translocates into the nucleus

19
Q

Nuclear localization signal (NLS)

A
  • used for targeting in cytosol-to-nucleus transport
  • specific sequence of AAs that are recognized by nuclear receptor proteins
20
Q

how is an NLS both necessary and sufficient for cytosol-to-nucleus targeting

A

Necessary: if it is mutated/not present, the protein will fail to target the nucleus
sufficient: if the sequence linked to a passenger protein is capable of redirecting the resulting fusion protein to the nucleus

21
Q

classic vs bipartite NLS

A

classic: short stretch of positive AA residues
bipartite: 2 short stretches of basic AAs and a 7-10 AA long spacer sequence

22
Q

Karyopherins

A
  • mobile proteins responsible for moving protein”cargo” across the nuclear envelope
  • large family of receptors responsible for moving macromolecules
    importins: move into the nucleus
    exportins: move out of the nucleus
23
Q

what is Ran protien

A
  • small G-protein involved in nuclear import and export
  • serve as molecular switches in the transport process
  • Ran-GTP = active
  • Ran-GDP = inactive
  • GTP hydrolysis provides the energy required for nucleocytoplasmic transport
24
Q

consequences of nuclear import failure

A
  • mutation in the NPC would lead to immediate cell death
  • mutation in the NLS may lead to disease (e.g. Sawyer syndrome)
25
Q

nuclear import of proteins: step 1

A

the NLS-containing cargo protein is recognized in the cytosol by importin-alpha
- then binds importin-beta to create a heterodimeric complex

26
Q

nuclear import of proteins step 2

A

the protein-importin receptor complex moves through the cytosol toward the nucleus via the cytoskeleton
- at the surface of the nucleus importin-beta binds to a cytoplasmic filament at the NPC

27
Q

nuclear import of proteins: step 3

A

the protein-importin receptor complex is translocated through the NPC
- protein-receptor complex interacts with FG domains of the FG Nups, this dissolves the FG-domain network to allow for translocation through the channel

28
Q

nuclear import of proteins: step 4

A

cargo-receptor complex associates with the nuclear basket and binds to Ran-GTP (via importin-B) resulting in its release from the NPC
- the complex disassembles in the nucleoplasm

29
Q

nuclear import of proteins: step 5

A

Ran-GTP bound importin-B moves back to the cytosol due to [Ran-GTP] gradient
- in the cytosol GTP on Ran-GTP is hydrolyzed by Ran-GAP1, and Ran-GDP is released from importin-B

30
Q

Ran gradient in the cell

A

[Ran-GTP] nucleus > [Ran-GTP] cytosol
[Ran-GDP] nucleus < [Ran-GDP] cytosol

31
Q

Ran accessory proteins

A

GAP = Ran-GAP1: cytosolic protein, promotes hydrolysis of Ran-GTP to Ran-GDP
GEF = RCC1: nuclear protein, promotes conversion of Ran-GDP to Ran-GTP

32
Q

nuclear export signal (NES)

A

specific AA sequence that is recognized by exportin to mediate targeting of the protein from the nucleus to the cytosol
- most common NES consists of a leucine-based motif

33
Q

what is the fate of importin-alpha in the nucleus?

A
  • rely on export signals (NES) to get back to the cytoplasm so it can be used for nuclear import again
  • gets out the same way ‘cargo’ proteins do
34
Q

Nuclear export of proteins: step 1

A

importin-alpha (or any other cargo protein) binds to exportin via its nuclear export signal (NES)

35
Q

Nuclear export of proteins: step 2

A

the importin-alpha-exportin complex binds Ran-GTP in the nucleus
- Ran-GTP promotes stable assembly of the importing-alpha-exportin complex

36
Q

nuclear export of proteins: step 3

A

importin alpha-exportin-Ran-GTP complex is transported via the NPC to the cytosol
- following the [Ran-GTP] gradient

37
Q

nuclear export of proteins: step 4

A

in the cytosol, GTP on Ran-GTP is hydrolyzed by Ran-GAP1

38
Q

nuclear export of proteins: step 5

A

Ran-GDP is released from exportin which causes the release of importin-alpha (or the NES containing cargo)
- exportin moves back to the nucleus (via importin) for another round of export

39
Q

Nuclear export of mRNA: step 1 (Ran-independent)

A

multiple dimers of NXF1 and NXT1 bind to processed mRNA - the NXF1/NXT1 dimer direct the RNA/protein complex to the NPC channel

40
Q

Nuclear export of mRNA: step 2 (Ran-independent)

A

NXF1/NXT1 transiently interact with FG-domains in FG Nups

41
Q

Nuclear export of mRNA: step 3 (Ran-independent)

A

as the mRNA moves through the NPC, an RNA helicase removes the NXF/NXT dimer in an ATP-dependent manner

MCB 2050 second half (6 decks)

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