Nucleus Flashcards
What are the general functions of the nucleus?
> storage of DNA
transcription of RNA from DNA
mechanical element
- stiffer than rest of cell so limiting factor in how squashed a cell can get
What are the benefits of compartmentalisation of genetic material in eukaryotes?
> protect DNA from reactions in cell - enzymes
increase SA for membrane-localised reactions
increase efficiency of transcription & synthesis
more ways to regulate gene expression
How many nuclei do cells have?
Most have 1
Red blood cells = 0
Skeletal muscle = many
Describe the nuclear membrane
Double membrane
- contiguous w/ lumen of ER
What is the ER the site of?
Protein production by ribosomes, protein folding & glycosylation
What is the theory for the evolution of the nucleus?
DNA in ancient prokaryotic cell was tethered to membrane until it became bound to membrane bound ribosomes
What are nuclear lamina?
> structure composed of nuclear lamins
> dense fibrillar network inside the nucleus
What are nuclear lamins?
> related to intermediate filaments
structural: form meshwork on inner nuclear membrane
functional: associate w/ DNA
2 types: A/C & B
What can mutations in lamins lead to?
‘Laminopathies’
–> muscular dystrophy & progeria (premature ageing)
What is the LINC complex?
Protein complex that links nuclear lamina w/ cytoskeleton
Inner membrane: SUN/KASH proteins
Outer membrane: nesprins
(connect to diff cytoskeleton elements)
(4 types w/ structural & functional roles)
What is the nuclear matrix?
Network of fibres found throughout the inside of a nucleus
- analogous to a cytoskeleton (‘nuceloskeleton’)
What is the nuclear matrix composed of?
> NuMa > Titin > Spectrin > PLF > Nuclear actin > Motor proteins
In what 2 forms is DNA arranged within the nucleus?
Chromosomes
= DNA-protein complexes specially packaged for cell division
Chromatin
= complex of DNA, RNA & proteins
What are the 2 types of chromatin?
> Euchromatin
> Heterochromatin
Describe euchromatin
= ‘true’ chromatin
- less densely packed
- majority of genome
- localised in central part of nucleus
Describe heterochromatin
= ‘different’ chromatin
- dark under microscope
- tightly packed condensed DNA
- localised around lamina & nucleoli
How is DNA organised within the nucleus?
Chromosomes exist in discrete territories within nucleus
What does localisation of DNA within the nucleus affect?
Whether gees are expressed (=transcribed into RNA)
Less RNA production in LADs (heterochromatin) than away from lamina (euchromatin)
What are LADs?
Lamina-associated domains
How does info get out of the nucleus?
Export of RNA & ribosomes
- mRN, rRNA, tRNA & assembled ribosomal units
Which proteins need importing into the nucleus?
RNA polymerase
Ribosomal proteins
Transcription factors
All other structural proteins (lamins, histones etc)
Through which structure do proteins move in & out of the nucleus?
Nuclear Pore Complex (NPC)
What are the features of NPCs?
> ring structure w/ 8-fold symmetry
30 diff types of nuclear pore proteins (Nups)
central channel is filled w/ FG-Nups that form a selective barrier to transport
What are FG-Nups?
Phenylatanine-glycine motifs - Nuclear pore proteins
What do NPCs have common ancestry with?
COP I, COP II & clathrin
What are the 2 types of movement through the NPC?
What types of molecules are transported these ways?
Passive transport
- small molecules & proteins
Facilitated transport
- RNAs, larger proteins, macromolecular complexes
Where does the energy for facilitated transport come from?
What else does facilitated transport require?
> ATP hydrolysis for mRNA
GTP hydrolysis for proteins, tRNA & ribosomes
Nuclear transport receptors = karyopherins
(= karyo + phero(‘bring’))
What is mRNP?
What does it include?
mRNA-protein complex
Proteins that process, cap & splice
Export factors
How is mRNP exported & imported?
How does ATP hydrolysis affect mRNP?
Via the NPC
Release of Dbp5 in cytoplasm & remodelling on mRNP
How are proteins imported into the nucleus?
- Karyopherins = AKA nuclear transport receptors = AKA importins
- ->bind to cargo proteins in cytoplasm - interact w/ FG-Nups
- release cargos in the nucleus
How do importins release their cargo in the nucleus?
Ran-GTP binds to importin-beta
–> conformational change
= complex breaks apart
How are proteins exported from the nucleus?
- Exportins bind cargo proteins in nucleus
- interact w/ FG-Nups
- release cargo in cytoplasm
How do exporting load their cargo in the nucleus?
Ran-GTP binds to exportin
–> conformation change
= complex assembly
Ran-GTP then hydrolysed to Ran-GDP in cytoplasm
What are GTPases?
What happens when they bind to GTP?
Molecular switches
Hydrolyse a phosphate group
–> forms GDP
What is GTP hydrolysis into GDP assisted by?
What is GTP loading after hydrolysis assisted by?
GAP proteins in cytosol
= GTPase activating proteins
GEFs in nucleus
= guanine nucleotide exchange factors
What are nuclear localisation signals (NLS)?
> Protein motifs approx 8 amino acids long
Can occur anywhere in protein
Many basic residues e.g. lysine, arginine, proline
Binds to importins
What are nuclear export signals (NES)?
> Sequence w/ 4 hydrophobic amino acids
e.g. LxxxLxxLxL (L= leucine, x = any other aa)
Binds to exportins
How can NLS & NES be modified?
Phosphorylation
Describe nucleocytoplasmic shuttling
- NLS of the transcription factor NF-kB is masked when bound to IkB in cytoplasm
- signal like cytokine (TNFa) triggers degradation of IkB
- NF-kB can be imported to nucleus
- NF-kB relocalises to cytoplasm via exportins binding to its NES
Which non-membrane-bound organelles are contained in the nucleus?
> Nucleoli
- cells can have 1 large nucleolus or many small nucleoli
> Cajal bodies
PML bodies
Speckles
What are the major functions of the nucleolus?
What are their other functions?
> Transcription of rRNAs
Assembly of ribosomal subunits
> sensing & responding to stress
cell cycle regulation
What is the nucleolus composed of?
> segments of 10 chromosomes encoding rRNA genes
proteins for processing rRNAs
ribosomal subunit proteins
snoRNA oligonucleotides that help process rRNAs
What are the functions of Cajal bodies?
What are they made of?
RNA processing
Genome organisation
Dense foci of coilin protein
What are the functions of PML bodies?
DNA repair
Cell proliferation
Programmed cell death
What are the functions of speckles?
Gene transcription
mRNA processing
What is the role of the Ran-GTP/GDP cycle?
Maintain gradient of import/export
