1. The mammalian exosome mediates the efficient degradation of mRNAs that contain AU-rich elements
EMBO J January 15; 21(1-2): 165–174.
Devi Mukherjee, Min Gao, J.Patrick O’Connor, Reinout Raijmakers, Ger Pruijn, Carol S. Lutz, and Jeffrey Wilusz
Exosome Journal Club
Mike 8/11

2. The enzyme and the control of eukaryotic mRNA turnover
substrate specificity
mechanisms
(determining substrate specificity)
(exosome is recruited to substrate RNA )
Exosome
RNA
The enzyme and the control of eukaryotic mRNA turnover
Nature Struct. Mol. Biol. 2004, 11,
1.ARE(AU-rich elements)-binding proteins TTP, RSAU and KSRP interact with exosome
and recruit it to mRNA.
2.Exosome subunit (PM/Scl-75) binds ARE.

3. The nonamer UUAUUUAUU is the key AU-rich sequence motif that mediates mRNA degradation.
1.Labile mRNAs that encode cytokine and immediate-early gene products often
contain AU-rich sequences within their 3' untranslated region (UTR).
2.UUAUUUAUU is the minimal AU-rich motif that effectively destabilizes mRNA.
Mol Cell Biol Apr;15(4):

4. AU binding proteins recruit the exosome to degrade ARE-containing mRNAs
Cell, Vol 107, , 16 November 2001
1.Mammalian exosome is required for rapid
degradation of ARE-containing RNAs .
2. ARE recognition requires certain ARE binding
proteins (KSRP,TTP, AUF1 and HuR) that can
interact with the exosome and recruit it to
unstable RNAs, thereby promoting their rapid
degradation.

5. 3’-5’exonucleolytic degradation is responsible for the decay of
RNA substrates following deadenylation in HeLa cytoplasmic
extracts.
Yeast
mRNA turnover intermediates
AUG UAA
Poly (G)
Phosphothioate
HeLa Cell
deadenylation
exosome
3’-5’ exonucleolytic decay is the major, if not exclusive, pathway for turnover of mRNA following deadenylation in HeLa cytoplasmic extracts.

6. Exosomal proteins are required for 3’-5’ exonucleolytic decay in HeLa cytoplasmic extracts.
Immunodepletetion experiments

7. Exosome activity is regulated by AU-rich elements in mammalian cells.

8. PM-Scl75 protein interacts with RNAs that contain AU-rich instability elements.
ARE-TNF- ARE (34)
ARE-GMCSF ARE(51)

9. PM-Scl75 protein interacts with RNAs that contain AU-rich instability elements.

10. Summery
1. 3’-5’exonucleolytic degradation is responsible for the decay of RNA substrates following deadenylation in HeLa cytoplasmic extracts. 2.Exosomal proteins are required for 3’- 5’exonucleolytic decay in HeLa cytoplasmic extracts. 3.Exosome activity is regulated by AU-rich elements in mammalian cells. 4.PM-Scl75 protein interacts with RNAs that contain AU-rich instability elements.
Model
1.Loading the exosome complex onto mRNA is
promoted by AREs in 3’- UTR.
2.Exosome may help promote mRNA
deadenylation.
3.mRNA is degraded rapidly by exosome.

11. 1. 5’- 3’exonuclease activities exist in HeLa cytoplasmic extracts
m7Gppp
AUG UAA
A 70
1. 5’- 3’exonuclease activities exist in HeLa cytoplasmic extracts
Capped and polyadenylated RNA substrates are very stable in HeLa cytoplasmic extracts
AUG UAA
A 60
m7Gppp
substrate : SV-A60 RNA