Nucleolar Stress Flashcards
What is the nucleus
Large organelle in eukayrotic cells that contains DNA organize into regions with different condensation
What is the nucleus encolosed by
A double membrane the nuclear envelope
What are nuclear bodies
Regions with high concentration of specific proteins and RNA that form distinct often roughly spherical structures within the nucleus
What is one type of nucleor body we know
HSF1and HSF2
Can a nucleus contain several nucleoli
YES
What are nucleoli responsible for
Ribosome biogenesis in cells and regulating cellular stress responses
What are the three steps of biogenesis of ribosomes
- Transcription of pre-rRNA
- Processiong of pre-rRNA
- Assembly of ribosome subunits
What are the three different nucleolus sub compartments
Fibrillar center
Dense fibrillar component
Granular components
What is the fibrillar center
Ribosomal RNA synthesis occurs it associated with DNA sequences that encodes for rRNA, RNA polymerase 1 machinery and UBF
What is UBF
Upstream binding factor is an essential component of the RNA pol 1 preinitation compelex
What is the dense fibrialr component
Contains nascent pre-rRNA transcripts pre-rRNA processing factors including snoRNAs and snoRNP proteins e.g. Fibrillarin and Nop58 The pre-RNA precursor undergoes chemical modifications here
What is the granular component
Contains ribosomal subunits in various stages of assembly a specific protein here is NPM1. Precursor ribosomal subunits are ready to be sent to the cytoplasm.
What is the nucleophosmin
Nuclear export chaperone and directs the nuclear export and directs the nuclear exoprt of the 40S and 60 S ribosomal subunits
What is the most RNA in cells and what comes after that
rRNA then tRNA and then mRNA and non coding RNA
How many subunits is a eukaryotic composed of
4 different rRNA molecules organized into a large subunit and small subunit
What does the small subunit consint of
Single rRNA 18s which is 33 proteins
What is large ribosomal consits of
(60S) 3 rRNAS (28s, 5.8s and 5s) assembled ribsome is 80s
What is pre-rRNA
What does it encode
A single transcription unit which encodes 18S 5.8S and 28S rRNA
What is the order of pre-RNA and what are the differences between species
always in the 5’-3’ order going from 18s 5.8S and 28S only thing that is different between species are the size of the sapcers in the pre-rRNA transcript
How are pre-rRNA genes arranged and fuction as waht
Tandem arrays sitting one by one beside each other and function as nucleolar organize which is where the nucleolus is localized
Where are the 5.8S,18S and 28S clustered
5 different human chromsomes 13,14,15,21,22
What is the transcription of pre-rRNA mediated by
RNA polymerase 1
Where is 5s rRNA gene and what transcripes it
Present as a single tandem on chromsome 1 and the transcription occurs in the nucleoplasm using RNA Polymerase 3
How many rRNAs are there in primary rRNA
3
How is the 3 split up
Methylation and pseudouridination of pre-rRNA which requires small nucleolar RNAS they hybridize to pre-rRNAs and bind enzymes that are responsible for the modification
What are snoRNA
Have a stem loop structure and associate with specific proteins to form small nucleolar ribonucleoproteins
What are the two major classes of snoRNAS
Box C+D and box H+ACA
What does box C+D associate with
methyl transferase fibriallarin which mediates methylation of specific riboses
What does H+ACA snoRNA assocaite with
Pseudorudine syntahse dyskerin which converts uridine to pseudorudine invovles rotation of the pyrimidine ring
What are the steps of ribosomal subunit assembly
- Specific ribosomal proteins and snoRNAs associate with the nascent pre-rRNA transcript and form precursor Pre-90s complex in nucleolus
- Pre-90s complex is cleaved into two prescursor subunits Pre-60s and Pre40s
- Both small and large subunits pass through nuclear pore complex and move to cytoplasm where mature and functional ribsomes are assembled
Does it require a lot of energy
GTPases and ATPases espically with the large subunit
Where are nucleoli formed
Clusters of ribosmal genes called Nucleolus Organizer Regions NORs
What are nucleoli
Dynamic structures: each mitosis nucleolus breaks up as a chromsomes condense and after mitosis nucleous reformes at the tips of several chromsomes
Where are these culsters
NOR
Usually on the short arm of chromsomes which contains genomic repeats/transcription units encoding rRNA
What happens during the M phase
nucleolus dissasembles and many nucleolar proteins associate with the surface
What remains associated with active rDNA during M-phase
UBF serving as a miotic bookmark and binding platform for the RNA Pol 1 machinery
What happens in G1 phase
FC/DFC/DC structures evolve around active transcrption units resulting in the formation of a few large nucleoli
What happens in S-phase
RNA pol 1 leave active rRNA genes to prevent collisions with replication machinerany leads to dissapperance of FC cells focus energy on synthesizing molecules for S-phase they do not want to make new ribosmes
What happens in G2 phase
Large fused nucleoli may contain poised/inactive rDNA units genes are duplicated but not all of them are active in preparation for mitosis
the formation of nucleolus depend on and where are they encoded
specefic types of RNA called aluRNA (glue to the nucleoli) encoded in the introns
What is the most abundant repetitive intronic elements
Alu repeats scattered throughout the human genome at sites where their insertion has not disrupted gene experession
What are alu transcrips synthesize by
RNA pol 2 and 3
What does aluRNA do
Stabiliz and form the nucleolus by binding together the proteins which make the nucleolus (nucleolin and nucleophosmin)
What happens if you treat cells with amanitin
It is a selective inhibitor of RNA pol 2 and 3 and induce the depletion of aluRNA dispersed nucelolus
Under normal conditions what is happening
There is a regular transcription activity and the nucleoli are doin their regular job making ribosomes
What happens under stress condtions
Signficant remodeling of nucleoli
What do environmental cues such as heat shock do
Induce expression of specific intergentic spacer which targets proteins containing a peptide code termed the nucleolar detentions sequence
What happens when there is an accumulation of IGS
Formation of a large subnucleolar structure termed detention center structural remodeling and transcriptional inactivation of nucleoli
What is the detention center
Central part of inactive nucleoli which leads to redistribution of nucleolar factors and movement of FC/DFC structures towards nuclolar periphery there is a relocalzation of proteins in the nucelolus under stress inhibit formation of ribosmes
What is the compostion of protiens in the nucleoli
30% ribsomes subunit biogenesis
70% are not releated to biogenesis but are important in regulating different aspects of stress singaling
What is the nucleolus being recognizes as
Key hub in cellular stress response by senesing and recating to various stimuli
What does nucleolar stress signaling pathway rely on
Dynamic binding and release of proteins in response to stress timuli
What is P53 known as
Transcription factor as the tumour suppressor gene or the guardian of genome
What are the domains of P53
- Two transactivation domains at the N-termins
- DNA binding domain
- Oligomerization domain resosible for tetramerization of p53
- C-terminal domian with nuclear export and nuclear localization signals
What is P53 inihibited by
MDM2 induces its degradation
What is the active form of p53
Tetramer
What is most frequently mutated gene in human cancer and in which domain
TP53 and DNA-binding domain
Why is p53 is low in normal conditions
Because of E3 ubiquitin ligase MDM2 and fast degradation in proteasomes
What is the P53 signaling pathway under stress conditions
DNA damage activates ATM kinase that phosphorylates p53 to stabalize it and MDM2
Once phosphorylated p53 tetramer bind response elements in p53 induced genes
What can p53 do once activated
Induce apoptosis stop cell divsion so cell have energy (maintain in G1 and G2) and induce DNA repair
What is another nucleor protein that can bind MDM2
p14ARF
What is happening in the nucelolus under normal growth conditions
Ribosomal DNA repeats are transcribed by RNA pol 1 making rRNA
Production of RP which interact with rRNAS into large and small subunits
Then released to nucleus into cytoplasm
Where is p53 translated under normal conditions
In the cytoplasm then goes into the nucleus where it interacts with MDM2 gets ubquiniated and then degraded back into the cytoplasm
What happens to the nucelolus under stress response
Ribosomal protiens and rRNAS are released from the nucelolus into the nucleoplasm and cytoplasm
What happens to the membrane of the nucelouls
During Stress
Dissasembly all of the protiens present in the nucelous are not going to make any more ribosomal subunits now they do other jobs and have a high affinity to bind MDM2
What happens to MDM2 under stress
Binds to ribsome portiens and result in relase of p53 which is available now to be phosphorylated and induces the activation of p53 target genes
What does RPL26 do
Binds p53 mRNA and enhances its translation increasing the levels of p53 in cells
