nucleus Flashcards
nucleolus/nucleoli
Sub-organelle
No membrane
Site of ribosome biogenesis (the process of creating ribosomes in a highly regulated manner)
Formed around regions of DNA encoding ribosomal RNA (rRNA)
Specifically tandem repeated clusters of rRNA genes – Nucleolar Organizer Regions (NOR).
* Hotspot of transcriptional activity (~80% of total RNA in cell is rRNA).
* Thus, nucleoli are genetically defined structures formed as a result of
making ribosomes.
nuclear bodies
Membraneless nuclear sub-compartments.
Highly dynamic in nature and in constant flux.
In most cases, concentrated regions of protein and RNA (ribonucleoprotein complexes).
Formation associated with transcriptional and RNA processing activity.
Generally speaking, their formation may enhance process efficiency and facilitate regulation.
Nucleosome
DNA + histone complex
Chromatin
Packaging of over 2 m of DNA within nucleus.
Chromatin structure is dynamic
(extended/condensed).
Chromatin structure determines gene
expression
Regulation of chromatin structure
Histone tails (N-term or C-term) extending from nucleosome can be targets of several post-translational modifications (PTMs).
Unacetylated: chromatin is highly condensed (transcriptionally inactive) –HETEROCHROMATIN
Acetylation – chromatin is less condensed, (transcriptionally active) –
EUCHROMATIN
Histone PTMs represents a “histone code” to determine gene expression.
Proteins that modify histones control chromatin structure and access of DNA to replication, transcriptional and repair machinery
Transcriptional Machinery
1) Transcriptional activators bind to DNA to recruit chromatin remodelling complexes to “open up” chromatin structure.
2a)They also recruit a protein bridge (mediator) to help recruit transcription factors to a promoter sequence.
2b) Mediator complex facilitates assembly of the pre-initiation complex that includes loading a RNA polymerase (RNA pol II) on DNA.
3) After initiation, transcription is paused by an elongation factor complex (NELF/DSIF).
4) Elongation pause is relieved by phosphorylation and remodeling of the elongation factors by a cdk/cyclinpair (P-TEFb).
Ribosome biogenesis
Ribosomes are ribonucleoprotein
complexes
Ribosomal RNA is first transcribed by RNA Pol I as a large transcript (pre-rRNA) that is then processed to 28S, 18S and 5.8S mature rRNA found in ribosomes.
Process similar to processing of pre-mRNA into mature mRNA.
5S transcribed in nucleoplasm by RNA Pol III and diffuses into nucleolus.
60S and 40S ribosomal subunits undergo a quality control check prior to export into cytoplasm (ie only correctly assembled subunits are exported).
Passage is through nuclear pores and mediated by nuclear export adaptors.
Final assembly into functional translation machinery (80S) occurs in cytoplasm.
Nuclear Pore Complex (NPC)
Spans both nuclear membranes.
Sole gateway in or out of nucleus
Allows passive diffusion by small molecules (<40 kDA)
Anything bigger needs to be guided through.
Pore is big enough to fit the larger of the ribosomal subunits (60S, ~25 nm).
Human NPCs are truly gigantic ~125 MDa in size.
* Yet only made up of ~ 30 different proteins termed nucleoporins (Nups)
Different Nups are repeated 8-, 16-, 32- or 64-times.
8-fold spoke symmetry to NPC structure.
Nuclear transport - general principles
- Localization signal on the cargo
- amino acid sequence recognized for transport in (import) or out (export) of nucleus
– Import Signal: Nuclear Localization Sequence (NLS)
– Export signal: Nuclear Export Sequence (NES)
- Generally localization sequences are functionally distinct from the nature of cargo.
– Cargo may have multiple localization signals - Delivery/Transporter System
- Recognise localization signal on cargo to form a cargo complex.
– Overcome size limit barrier of NPC to allow movement through the pore.
– Transporters for nuclear import – Importins
– Transporters for nuclear export – Exportins.
– Importin/Exportin are receptors for FG-Nups - Control system for delivery
- Utilizes a GTPase switch
- “Activity” in this case is binding of cargo transporter (importin/exportin)
- In the case of nuclear transport, the key GTPase switch is a small Gprotein called Ran-GTP/GDP.
- Ran-GTP and Ran-GDP (23 kDa) can randomly diffuse through nuclear pore
Nuclear Import Mechanism
- Ran-GTPS bind to importins
- importins then lose their cargo
- the ran-importin then diffuse back to the cytoplasm
- cytoplasm converts Ran-GTP to Ran-GDP - so they release importin
- importins are recycled
- Ran-GDP goes back to the nucleus to become Ran-GTP again
nuclear export mechanism
same as the import except instead of losing the cargo when binding, it keeps it and releases it into the cytoplasm later on
cargo movement is unidirectional
