the nucleus Flashcards
1
Q
2 main functions of the nucleus
A
- compartmentalization of the cellular genome and its activities
- DNA replication, transcription, RNA processing - coordination of cellular events
- control of metabolism, protein synthesis, reproduction
2
Q
what components make up the nuclear envelope
A
- nuclear membrane
- nuclear lamina
- nuclear pores
3
Q
what does nuclear content consist of (inside)
A
- chromatin
- nucleoplasm
- nucleolus
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
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)
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
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
8
Q
Nuclear speckles
A
- subdomains where mRNA splicing factories are concentrated
- often located next to transcription factories
9
Q
the nucleolus
A
- NOT membrane-bound
- site of ribosome biogenesis
- initial stages or ribosomal subunit assembly (rRNA + protein)
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
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
12
Q
NPC structure
A
- octagonal symmetry
- structural and membrane nucleoporins
- FG domains
- cytoplasmic filaments
- nuclear basket
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
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
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
16
Q
nuclear basket
A
- located on the nuclear side of the NPC
- involved in nuclear receptor-cargo protein import and export
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
nuclear import of proteins: step 1
the NLS-containing cargo protein is recognized in the cytosol by importin-alpha
- then binds importin-beta to create a heterodimeric complex
26
nuclear import of proteins step 2
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
nuclear import of proteins: step 3
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
nuclear import of proteins: step 4
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
nuclear import of proteins: step 5
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
Ran gradient in the cell
[Ran-GTP] nucleus > [Ran-GTP] cytosol
[Ran-GDP] nucleus < [Ran-GDP] cytosol
31
Ran accessory proteins
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
nuclear export signal (NES)
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
what is the fate of importin-alpha in the nucleus?
- 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
Nuclear export of proteins: step 1
importin-alpha (or any other cargo protein) binds to exportin via its nuclear export signal (NES)
35
Nuclear export of proteins: step 2
the importin-alpha-exportin complex binds Ran-GTP in the nucleus
- Ran-GTP promotes stable assembly of the importing-alpha-exportin complex
36
nuclear export of proteins: step 3
importin alpha-exportin-Ran-GTP complex is transported via the NPC to the cytosol
- following the [Ran-GTP] gradient
37
nuclear export of proteins: step 4
in the cytosol, GTP on Ran-GTP is hydrolyzed by Ran-GAP1
38
nuclear export of proteins: step 5
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
Nuclear export of mRNA: step 1 (Ran-independent)
multiple dimers of NXF1 and NXT1 bind to processed mRNA - the NXF1/NXT1 dimer direct the RNA/protein complex to the NPC channel
40
Nuclear export of mRNA: step 2 (Ran-independent)
NXF1/NXT1 transiently interact with FG-domains in FG Nups
41
Nuclear export of mRNA: step 3 (Ran-independent)
as the mRNA moves through the NPC, an RNA helicase removes the NXF/NXT dimer in an ATP-dependent manner
