Moving into the nucleus Flashcards
What separates eukaryotes and prokaryotes?
Ability of a cell to compartmentalise distinct processes within organelles
What is a eukaryotic cell?
A cell with distinct membrane bound nucleus and other organelles enclosed in a membrane
What is the purpose of membrane bound compartments?
Modulate interactions between proteins
Allow regulation of the local environment
Provides a distinct environment
Provides a level of control
What are the 3 main exchange mechanisms?
Gated transport
Transmembrane transport
Vesciular transport
What is gated transport?
Occurs through pore complexes
What is transmembrane transport?
Involves direct passage across intact membranes
What is vesicular transport?
Does not involve passage across membranes
What can be caused by mislocalised proteins?
Associated with human diseases such as swyers syndrome
What is Swyer Syndrome?
Mutation in SRY gene resulting in gonads failing to differentiate into testes. Therefore they are genetically male but with external female sex organs
Caused by mutation in SRY gene responsible for a localisation of a protein into the nucleus
What is the structure of the nuclear envelope?
Phospholipid bilayer
Outer membrane continuous with the ER
Inner membrane has underlying filaments called nuclear lamina
What is the nuclear pore complex?
Largest protein complex in the cell
3000 NPCs per nucleus
What does the nuclear pore complex do?
Allows the free movement of small molecules and selective transport of larger proteins (acts as a molecular sieve)
What happens to the NPC during mitosis?
NPCs are dismantled and then reassembled during and after mitosis
What is the structure of the nuclear pore complex?
Comprises of 500 subunits of 30 individual proteins
8 fold symmetric structure
Centre structure encases the transport channel and functions as the sieve
Peripheral structure connects the NPC to the local environment
What are Nups?
Nucleoporins
What are the different classes of Nups?
Transmembrane ring Nups
Core scaffold Nups
FG Nups
Linker Nups
What do the transmembrane ring and Core scaffold Nups do?
Anchor into the nucleus envelope
Anchor the NPC into the membrane
What do the FG Nups do?
Line the channel
Unstructured Nups which form a hydrogel that functions as a sieve.
What doe the linker Nups do?
Anchor the FG Nups into their positions
What do the cytoplasmic filaments do?
Link NPC to the cytoplasm
What do the baskets do?
Link NPC with the nucleus
What regulates import/export of proteins across nuclear membranes?
Cycle of interactions between:
Protein cargo
Nuclear transport receptors (NTRs)
GTPase Ran
How are proteins imported and exported by NTRs?
- Cargo is recognised by NTRs
- NTR-cargo docks onto NPC via FG Nups and the cargo is translocated through the NPC
- NTR-cargo dissociate
How do proteins target the nucleus?
Nuclear localisation sequence (NLS)
What is the nuclear localisation sequence?
Run of >5 basic amino acids which direct cytosolic proteins to the nucleus.
NLS can be found anywhere in a proteins sequence
What are the nuclear importins?
Heterodimers of importin alpha and importin beta
How do the nuclear importins work to import cargo into the nucleus?
- Importin alpha recognises the NLS on cargo proteins
- Importin beta interacts with the NPC via FG repeats within nucleoporin proteins
- Movement into the pore is random
- Directionality of the cargo is determined by a RanGTP gradient allowing the importin into the nucleus
- Once in the nucleus, RanGTP binds to importin beta and causes the release of the cargo to travel to its desired site
What determines the directionality of cargo import into the nucleus?
A RanGTP gradient
What does the RanGTP cycle allow for?
Recycling of importins back into the cytoplasm
Export of proteins into the cytoplasm
How does the RanGTP gradient allow for the recycling of importins back out of the nucleus and export of proteins with cargo?
- Once in the nucleus RanGTP competes with the cargo and bind to the importin.
- Concentration of RanGTP is greater in nucleus (RanGDP is greater in cytoplasm) than cytoplasm and so will diffuse out of the nucleus taking the importin with it.
How is the RanGTP gradient set up?
Concentration of RanGTP is higher in the nucleus than cytoplasm
Concentration of RanGDP is higher in the cytoplasm than nucleus
This sets up a gradient.
What are the nucleus exportins?
Crm1 - Protein exportin
NXF1 - mRNA exportin
What localisation sequence do exportins require?
Nuclear export sequence (NES) which is a leucine rich region.
What do Exportins require to move cargo out of the nucleus?
Require a RanGTP gradient for both interaction with cargo and then their movement out of the nucleus.
How is Ran regulated?
Proteins that regulate ran are localised in the nucleus and cytoplasm to promote a constant state of Ran in its either GTP/GDP state.
What protein regulates Ran in its GTP state in the nucleus?
Ran-GEF promotes loading of GTP onto Ran
What protein regulates Ran in its GDP state in the nucleus?
Ran-GAP promotes GDP loading onto Ran via GTP hydrolysis.
What concentrates Ran-GEF to the nucleus?
Eg. RCC1 (A Ran-GEF molecule) is bound to chromatin in the nucleus
What concentrates Ran-GAP to the cytoplasm?
Ran-GAP is bound to cytoplasmic filaments.
What is an important regulator of import?
Phosphorylation of Cargo/protein
How does phosphorylation regulate import?
Increase import
Decrease import
What are the ways that phosphorylation can increase import into the nucleus?
Increasing binding affinity of cargo to importin alpha (EBNA-1)
Enhanced recognition of cargo by importin alpha (SV40)
Induced conformational change exposing NLS (STAT1 transcription factor)
What are the ways that phosphorylation can decrease import into the nucleus?
NLS-phosphorylation induces cytoplasmic retention (NFAT transcription factor)
What are the processes that must occur before mRNA can be exported out of the nucleus?
Capping - capping binding complex (CBC)
Splicing - exon-junction complex (EJC)
3’ end processing
How does mRNA export occur?
- ALY recruits NXF1 which is the main export factor
- NXF1 binds to FG Nups lining the NPC
- RanGTP gradient is not utilised
- GLE1 and DDX19 (cytoplasmic part of nuclear pore) provide directionality by removing export factors
- ALY is recycled back to the nucleus via importins.
