RNA maturation transport & localization
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Transcript of RNA maturation transport & localization
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RNA maturation transport & localization
RNA export to the cytoplasm: model systems
RNA degradation
Links between RNA processing, transport, degradationRNA localization in the cytoplasm
RNA processing: brief overview
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Evidence for checkpoints?mRNA
transport
All eukaryotic mRNAs are processed
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Specific process for Pol II transcriptsnuclear CBC binds to m7G-cap: role in splicing, transport, stability (exchanged in cytoplasm)
Function in transport, stability, translation
1. Capping
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CPSF: cleavage and polyadenylation specificity factor
CStF: cleavage stimulatory factor
CF: cleavage factor
PAP: polyA polymerase
PAB: polyA binding protein
Function in transport, stability, translation
2. Polyadenylation
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3. Splicing
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QuickTime™ and aAnimation decompressor
are needed to see this picture.
3. Splicing
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Splicing is mediated by snRNPs
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Compartments inside the nucleus
1. Nucleolus
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2. Perichromatin granule clusters
Carter et al., Science (1993) 259: 1330
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Balbiani Rings (Chironomus tentans)
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BR RNP maturation
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BR RNP translocation
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BR RNP translocation
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Retroviruses as mRNA export models
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Gene location is influenced by gene activity
Casolari et al., 2004
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SUMMARY
I. Multiple steps of mRNA biosynthesis are tightly coupled
II. Mex67/TAP is one major mRNA export factorBinding to mRNA may already occur at the site of transcription
III. Many questions remain- How are mRNA substrates released in the cytoplasm? - Are there multiple mRNA export pathways?- How is processing and transport mechanistically “coupled”?- Is mRNA export regulated?etc.
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RNA half-lives vary greatly but are highly coordinated
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RNA half-lives vary greatly but are highly coordinated
Wang et al., PNAS 2002
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AUUUA element regulates half-life
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A.
B.Casein mRNA prolactin
+
-
30,000 mRNA/cell
300 mRNA/cell
No change in transcription
Examples of regulated mRNA turnover
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mRNA DEGRADATION
mRNA DECAY NMD
‘turnover’ ‘surveillance’
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mRNA DECAY
AAAm7Gppp
poly A shortening Deadenylase complex
AAAAAAAAAAAAAAAAm7Gppp
Decapitation Decapping enzyme (DCP1 complex)
AAA
5’-3’ exonucleolytic cleavage Xrn1 complex
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Decay factors localize to cytoplasmic processing bodies (P bodies)
Sheth et al. Science 2003
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mRNA activity is regulated by multiple factors
Active mRNAs Inactive mRNAs
Storage
Decay
Transport
Translation
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From Neu-Yilik et al. (2001) EMBO 20:532-540
Position of nonsense codon affects mRNA amounts
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m7Gppp
Stop in penultimate exon/5’ of splicing mark
AAAAAAAAAAAAAAAA
Decapitation Decapping enzyme (DCP1 complex)
5’-3’ exonucleolytic cleavage Xrn1 complex
Nonsense Mediated Decay
AAAAAAAAAAAAAAAA
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RNA localizationmRNA can be localized to subcellular compartmentsby actin or tubulin-dependent processesExamples:
Xenopus: Vg1 mRNA (TGFb) to vegetal pole
Drosophila: nanos, oskar mRNA (posterior) and bicoid (anterior)
(requires mRNA binding protein staufen)
(requires staufen and miranda)
prospero (into ganglion of mother cells; neuroblast TF)
Yeast: Ash1 mRNA to daughter cell
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Examples of localized mRNAs in various systems
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lamellipodia staining perinuclear staining in myotubes
3’ UTR determins localization of many mRNAs
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Bertrand et al., Mol Cell (98) 2:437-445
Ash1 mRNA specifically localizes to new daughter cells
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Mechanism of Ash1 mRNA localization
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SUMMARY
I. mRNA decay- regulated and non-regulated turn-over but apparently coordinated- ordered pathways (e.g. deadenylation, decapping, exonucleolytic degradation)- cross-talk between translation and turnover- important regulation via non-coding RNAs- turnover occurs in specific cytoplasmic compartments- NMD: recognition of premature stop codons
II. Cytoplasmic mRNA localization- ZIP code in 3’ UTR- both actin and tubulin-mediated - yeast mating type switch as a model: Ash1 mRNA localization (via 3’ UTR, She2/3, Myo4 and actin cables)