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Trasporto dell'mRNA
Transport through the nuclear pores
• The NLS and NES consist of short sequences that are necessary and sufficient for proteins to be transported through the nuclear pores.
• Transport receptors have the dual properties of recognizing NLS or NES sequences and binding to the nuclear pore.
• The direction of transport is controlled by the state of the monomeric G protein, Ran.
• The nucleus contains Ran-GTP, which stabilizes export complexes, while the cytosol contains Ran-GDP, which stabilizes import complexes.
• The mechanism of movement does not involve a motor.
Elementi regolativi sull'mRNA
Elementi regolativi sull'mRNA
Fattori che influenzano la stabilità dell'mRNA
Summary of methods to assess mRNA stability in eukaryotic cells
Method Advantage Disadvantage Comments
Pulse-chase labeling with 3H-U
measurement of "true" chemical half life
low sensitivityfor high abundance,
slow turnover mRNAs
Injection of in vitro transcribed 32P-
RNA
measurement of "true" chemical half life
lack of cellular RNA modifications, labour
intensive
oocytes can differ from somatic cells
Inducible promoterrelatively rapid
inductioninduction may alter cell
physiologyHsp70 and myc
promoter
Pharmacological transcription block
can be applied to all genes, rapid onset
block
perturbation of cellular metabolism
Actinomycin D, and DRB most commonly
used
Comparison of transcription rate and steady state
mRNA level
can be applied to all genes, useful
screening procedure
mRNA stability is not directly measured
should only be used in combination with
another method
In vitro RNA degradation
easy, identification of intermediates,
purification of trans-acting factors
difficult to establish
physiological relevance and
specificity must be established
Analisi della stabilità dell'mRNA
Degradazione dipendente da deadenilazione
Meccanismi alternativi di degradazione
mRNA degradative activities in mammalian cells
Deadenylation
•PARN •CCR4/CAF1/NOT complex•PAN2/PAN3 complex
•Decapping•DCP2 which binds RNA as a prerequisite for cap recognition. •DCP1 augments DCP2 activity •LSM (SM-LIKE) PROTEINS augment DCP2 activity
5’ -to-3’ exonuclease activity•XRN1 is a proven 5’ -to-3’ exonuclease that localizes to the cytoplasm.•RAT1/XRN2 is only thought to be a 5’ -to-3’ exonuclease on the basis of its similarity to the yeast orthologue.
3’ -to-5’ exonuclease activity•Exosome (six RNase-PH-DOMAIN components, PM/SCL75,MTR3,RRP41, RRP42, RRP43 and RRP46; three S1 and KH RNA-binding components,RRP4, RRP40 and CSL4; the RNASE D-like components PM/SCL100; the putative helicaseKIAA0053; and a protein that is phosphorylated in the M phase of the cell)
PMR1-like activity•Polysomal ribonuclease 1 (PMR1) is a polysome-associated mRNA endonuclease
Elementi di regolazione della stabilità dell'mRNA
Deadenilazione e traduzione
Elementi ARE (AU rich element)
ARE-binding proteinsProtein kDa Motif Expression site ARE Function
AUF1 37,40,42,45 RRM Ubiquitous c-myc, c-fos, GM-CSF mRNA destab.
AUBF ND ND T cells c-fos, INF, IL-3 v-myc, GM-CSF, (AUUUA)n
ARE-binding corr. with mRNA stab.
AU-A 34 ND T cells TNF, GM-CSF, c-myc ND
AU-B 30
AU-C 43
hnRNPA1 36 RRM Human PBMCs GM-CSF, IL-2, c-myc ND
hnRNPC 43
Elav-like 36–40 RRM Ubiquitous, nervous system c-myc, c-fos,TNF-a,GM-CSF mRNA stab.
HuR
HuD
HuC
Hel-N1
TIAR 40, 42 RRM Brain, spleen, lung, liver,testis TNF, GM-CSF Transl. inhib.
TIA-1 Brain, spleen, testis
TTP 44 Cys3His Fibroblasts, macrophages TNF, IL-3 GM-CSF mRNA destab.
KSRP 78 KH Neural cells and other types c-fos mRNA destab.
•AUBF, AU binding factor ; AU-A, AU binding factor-A ; AU-B, AU binding factor-B ; AU-C, AU binding factor-C ; hnRNP, heterogeneous nuclear ribonucleoprotein ; KH, hnRNP-K homology domain; KSRP, KH-type splicing regulatory protein 1; ND, not determined; PBMC, peripheral blood mononuclear cell.
ARE-binding proteins
Modello sul ruolo delle ARE e ARE-BP
Degradazione dipendente dalle ARE e ARE-BP
Degradazione mediata da endoribonucleasi
AUG UAG
mGppp AAAAAAAAAAAAAAAA
PTC
NMD (Nonsense Mediated Decay)
NMD (Nonsense Mediated Decay)
PTC nei mammiferi
Formazione EJC
Non-sense Mediated Decay (NMD): complesso EJC
Non-sense Mediated Decay (NMD)
"Pioneer round" of translation
32
Meccanismo dell’NMD
UAG (PTC)AUG STOP
mGppp AAAAAAAAAAAAAAAA3 3
2
mGpp p AAAAAAAAAAAAAAAA3
UAG
AAAAAAAAAAAAAAAAmGppp
UAG
3 322 1
RF
Degradazione normale
Degradazione NMD
Non-Stop Mediated Decay (NSMD)
Analisi della stabilità
dell'mRNA