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MCB*2050 - Lectures 20-22
Nucleus, Nuclear Import and Export, Nuclear Content
38
Microbiology
Undergraduate 2
03/27/2014

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Cards

Term
What is the nuclear envelope?
Definition
- boundary between the cytosol and nucleus
Term
What are nuclear pores?
Definition
- "doorways" in the nuclear envelope
Term
What are the two main functions of the nucleus?
Definition
1. compartmentalization of cellular genome and activities
2. coordination of cellular activities
Term
What is included in the nuclear envelope?
Definition
1. nuclear membrane
2. nuclear lamina
3. nuclear pores
Term
What is included in the nuclear content?
Definition
1. chromatin
2. nucleoplasm
3. nuclear matrix
4. nucleolus
Term
What are the three domains of the nucleus?
Definition
1. outer membrane
2. inner membrane
3. intermembrane space
Term
What occurs at the outer membrane?
Definition
- ribosome binding
Term
What is the outer membrane continuous with?
Definition
- the rough ER
Term
What important structures does the inner membrane contain?
Definition
- integral membrane proteins that connect the nuclear lamina
Term
What is the intermembrane space continuous with?
Definition
- the ER lumen
Term
What are the functions of the nuclear envelope?
Definition
1. separates nuclear content from cytoplasm
2. acts as a selective barrier
3. binds nuclear lamina
Term
Explain how the nuclear envelope acts as a selective barrier?
Definition
- allows regulated passage of molecules between the nucleus and cytosol
Term
Describe the structure of the nuclear lamina
Definition
- thin meshwork of long filament-like proteins
Term
Where is the nuclear lamina located?
Definition
- bound to the inner surface of the nuclear envelope
Term
What are the functions of the nuclear lamina?
Definition
1. support structure for nuclear envelope
2. scaffold for chromatin and nuclear matrix attachment
Term
What are nuclear pore complexes (NPCs)?
Definition
- protein structures that fill nuclear pores
- made up of nucleoporins (NUPs)
Term
Describe the structure of a nuclear pore complex.
Definition
- octagonal symmetry organized around a large channel
- central scaffold anchors complex to nuclear envelope
Term
What is the inner surface of the NPC channel lined with?
Definition
- filament-like proteins
Term
What do the FG domains form?
Definition
- form a hydrophobic "mesh" that limits diffusion of large macromolecules (>30nm diameter)
Term
What are the main structures involved in nuclear receptor-cargo protein recognition and import?
Definition
- cytoplasmic filaments
Term
What are the 2 major functions of nuclear pore complexes?
Definition
1. passive diffusion of small molecules
2. to regulate movement of larger molecules
Term
What is a nuclear localization signal (NLS)?
Definition
- specific stretch of AA's that are recognized by nuclear receptor proteins
Term
Why is a nuclear localization signal needed?
Definition
- serves as a zip code to mediate targeting of protein from cytosol to nucleus
Term
What is the difference between a classic NLS and a bipartite NLS?
Definition
- classic: consist of a short stretch of basic AA
- bipartite: composed of two short basic AA sequences with a spacer sequence in between
Term
What are other proteins that are necessary for nucleoplasmic transport?
Definition
- transport proteins aka karyoferins
- small GTP-binding proteins (ex. Ran)
Term
Outline step 1 of nuclear import.
Definition
1. nascent NLS-containing cargo proteins are recognized by importin alpha and beta
2. importin alpha recognizes and binds to the basic residues in the NLS
Term
Outline step 2 in nuclear import.
Definition
1. cargo protein-importin complex moves through the cytosol towards nucleus
2. at surface of nucleus, importin beta binds to a cytoplasmic filament at the NPC
Term
Outline step 3 in nuclear import.
Definition
1. complex is translocated through aqueous central channel of NPC
2. interacts with FG domains of the NUPS in the central channel
3. interaction dissolves the domain network and allows translocation
Term
Outline step 4 in nuclear transport.
Definition
1. complex associates with nuclear basket on inner surface of NPC
2. binds to active Ran-GTP (via importin beta) and results in release from NPC and disassembly into nucleoplasm
Term
Outline the first part of step 5 of nuclear import.
Definition
1. Ran-GTP and importin beta moves back to cytosol due to low cytosolic [Ran-GTP]
2. in the cytosol, GTP is hydrolyzed (via GAP1) to GDP
3. Ran-GDP is released from importin beta
4. Ran-GDP moves back into the nucleus due to low nuclear [Ran-GDP] gradient
4. GDP converted back to Ran-GTP by RCC1
Term
Outline the second part of step 5 in nuclear import.
Definition
1. importin alpha binds to exportin
2. complex binds Ran-GTP
3. full complex is transported into cytosol due to [Ran-GTP] gradient
4. in the cytosol, GAP1 hydrolyzes GTP to GDP
5. Ran-GDP is released from export in, causes release of importin alpha
6. Ran-GDP moves back into nucleus and is converted to GTP by RCC1
7. exportin also moves back into nucleus
Term
Describe piggybacking.
Definition
- some proteins lack an NLS so they bind to a protein that DOES contain an NLS and piggybacks into the nucleus
Term
What is the nucleoplasm?
Definition
- highly organized
- fluid-filled interior of the nucleus
- consists of >30 specialized regions with specific functions
Term
What is the nuclear matrix?
Definition
- insoluble fibrillar-like protein network
- distributed throughout the nucleoplasm
Term
What are the functions of the nuclear matrix?
Definition
1. structure - maintain overall 3D structure of nucleus
2. scaffold - organizing nuclear subdomains
Term
What are nuclear speckles?
Definition
- subdomains where mRNA splicing factors are concentrated
Term
The size and number of nucleoli depend on what?
Definition
- metabolic activity of the cell
Term
What is the main function of the nucleolus?
Definition
- ribosome biogenesis
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