Chapter 20 - mRNA stability Flashcards
mRNAs have characteristic structures that are related to
their stability in the cell
Stability of mRNA is related to
ribonuclease activity
exoribonucleases and endoribonucleases
mRNA-specific stability information is encoded in
cis-sequences
stability is measured in
half-life
mRNA stability can range
100-fold or more
Eukaryotic mRNA exists as
mRNPs
RBP
Ribonucleoprotein particles than contain RNA and RNA-binding proteins (RBPs)
Eukaryotic mRNAs are associated with a distinct combination of
RBPs that serve to classify the mRNA
Eukaryotic mRNAs are protected from degradation by
3’ tail and 5’ cap
Most mRNA degradation is initiated by removing the
3’ tail
Tail is gradually shortened as soon as it enters the cytoplasm by
poly(A) nucleases
polyA nucleases
PAN2/3 slow initial degradation
CCR4-NOT quickly removes remaining tail
Nuclear CCR4-NOT complexes promote
transcription elongation
5’ to 3’ decay pathway
Digestion of the poly(A) tail down to 10-12 nt
Lsm1-7 decapping enhancer binds to short poly(A) tail
Lsm1-7 activates the decapping reaction on the 5’ end
Removal of the cap produces a 5’ monophosphorylated RNA
5’ to 3’ Xrn1 exonuclease rapidly degrades the mRNA
The 5’ cap is usually resistant to decapping while it is being translated because
it is bound to a cytoplasmic cap-binding protein
Translation and decapping machineries compete for
the cap
initiation of translation involves an
interaction between the PABPs and the eIF4F initiation complex at the 5’ end
Removal of the tail and release of PABPs destabilizes the eIF4F-cap interaction
Cap is more exposed to decapping enzymes
3’ to 5’ decay pathway
Digestion of the poly(A) tail down to 10-12 nt triggers exosome action
The exosome is a multiprotein complex that contains a 3’to5’ exonuclease
The exosome degrades the mRNA from the 3’ end
Deadenylation independent decapping and degradation
Decapping and Xrn1 digestion proceeds in the presence of a full-length tail
Requires a mechanism to prevent eIF4F binding to the cap
RPS28B mRNA
S28 is the protein product of this mRNA
Small ribosomal subunit protein
Stem-loop in 3’-UTR of RPS28B mRNA recruits a decapping enhancer
Only recruited when bound by S28 protein
Destabilizes eIF4F
Cell-cycle regulated histone mRNAs in mammalian cells
High levels required during early S phase and should be rapidly degraded in late S
Histone expression should be reduced after DNA and chromatin synthesis is complete
These mRNAs have a 3’ structure like that of the stem-loop observed in bacterial mRNAs
No poly-A tail
Have a short poly(U) tail added on the 3’ end of the mRNA in late S
“Platform” for Lsm1-7
Standard 3’5’ poly-A dependent decay pathways are then activated
PolyA removal-independent degredation
- Deadenylation independent decapping and degradation
- Cell-cycle regulated histone mRNAs in mammalian cells
