rna maturation transport & localization rna export to the cytoplasm: model systems rna...
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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
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
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
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.
A.
B.Casein mRNA prolactin
+
-
30,000 mRNA/cell
300 mRNA/cell
No change in transcription
Examples of regulated mRNA turnover
mRNA DECAY
AAAm7Gppp
poly A shortening Deadenylase complex
AAAAAAAAAAAAAAAAm7Gppp
Decapitation Decapping enzyme (DCP1 complex)
AAA
5’-3’ exonucleolytic cleavage Xrn1 complex
mRNA activity is regulated by multiple factors
Active mRNAs Inactive mRNAs
Storage
Decay
Transport
Translation
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
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
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)