After undergoing rounds of translation, mRNA is normally destroyed by the deadenylation-dependent pathway. Though the trigger is unclear, deadenylation likely proceeds in two steps: one catalyzed by the PAN2-PAN3 complex that shortens the poly(A) tail from about 200 adenosine residues to about 80 residues and one catalyzed by the CCR4-NOT complex or by the PARN enzyme that shortens the tail to about 10-15 residues. After deadenylation the mRNA is then hydrolyzed by either the 5' to 3' pathway or the 3' to 5' pathway. It is unknown what determinants target a mRNA to one pathway or the other. The 5' to 3' pathway is initiated by binding of the Lsm1-7 complex to the 3' oligoadenylate tail followed by decapping by the DCP1-DCP2 complex. The 5' to 3' exoribonuclease XRN1 then hydrolyzes the remaining RNA. The 3' to 5' pathway is initiated by the exosome complex at the 3' end of the mRNA. The exosome processively hydrolyzes the mRNA from 3' to 5', leaving only a capped oligoribonucleotide. The cap is then removed by the scavenging decapping enzyme DCPS.
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The human CCR4-NOT complex contains 7 core subunits: CNOT1, CNOT2, CNOT3, CNOT9/RCD1, CNOT10, TAB182, and C2ORF29. Complexes contain either CNOT7 or CNOT8 (with CNOT8-containing complexes apparently involved in nuclear RNA splicing and CNOT7-containing complexes involved in cytoplasmic mRNA decay) and CNOT6 or CNOT6L. CNOT6 and CNOT6L are catalytic exoribonucleases. CNOT7 and CNOT8 also have ribonuclease activity. CNOT1 is the largest subunit and, based on yeast two-hybrid assays, interacts with CNOT2, CNOT7, CNOT8, and CNOT9, thus acting as a scaffold.
The exosome complex comprises a ring and associated subunits. The ring contains EXOSC2, EXOSC7, EXOSC8, EXOSC9, EXOSC5, and EXOSC6. The subunits EXOSC1, EXOSC2, EXOSC3, and RRP44 bind the ring. The catalytic ribonuclease site is located in RRP44, which yields ribonucleotides having 5'-monophosphates.
mRNA's that are ready for translation have a circular structure caused by interaction between PABP bound to the 3' poly(A) tail and the eIF4E-eIF4G-PAIP complex bound to the 7-methylguanosine cap. The interaction between poly(A)-PABP and the eIF4G-eIF4E complex stimulates affinity of eIF4E for the cap and improves translation.
The XRN1 exoribonuclease hydrolyzes decapped mRNA from 5' to 3' and yields ribonucleotides having 5'-monophosphates. In yeast Xrn1 associates with the Lsm1-7 complex.
The Lsm1-7 complex forms a heptameric ring that binds the 3' oligoadenylated ends of mRNAs that have been deadenylated. The bound Lsm1-7 may prevent access of the exosome (a 3' to 5' exonuclease) to the 3' end and thereby direct the mRNA to the 5' to 3' exonuclease pathway. The yeast Lsm1-7 complex has a preference for oligoadenylated RNA compared to polyadenylated RNA, however other determinants of binding by Lsm1-7 are unknown.
The scavenging nuclease DCPS hydrolyzes the triphosphate bond between the 7-methylguanosine cap and the remaining oligoribonucleotide body of the mRNA. The products are 7-methylguanosine 5'-monophosphate and an oligoribonucleotide with a 5'-diphosphate.
The DCP1-DCP2 decapping complex binds the 7-methylguanosine cap of mRNA and hydrolyzes the triphosphate bond to yield 7-methylguanosine 5'-diphosphate and RNA with 5'-monophosphate. The DCP2 subunit of the complex catalyzes the hydrolysis. DCP2 has higher affinity for some subsets of mRNA.
The exosome complex hydrolyzes capped, deadenylated mRNA from 3' to 5' and yields ribonucleotides having 5'-monophosphates. In yeast the Ski2-Ski3-Ski8 complex assists degradation by the exosome complex, however little is known about the function of the homologous Ski complex in mammals. Although many exosomal components contain exonuclease signatures, only two components have been shown to degrade RNA. Rrp6/PMSCL-100 has been shown to be involved in the 3’-5’ decay of nuclear mRNAs in yeast. Rrp6 may also function in the absence of the core exosomal components. The Rrp44/dDis3 component of the core exosome has been shown to possess both 3’-5’ exonuclease activity along with endonuclease activity via its PIN domain.
The PAN2-PAN3 exoribonuclease complex hydrolyzes the poly(A) tail of a mRNA, shortening the tail from about 200 adenosine residues to about 80 adenosine residues and yielding adenosine 5'-monophosphate. PAN2 is the exoribonuclease component of the complex; PAN3 is required for cellular localization. The poly(A)-binding protein (PABP) interacts with PAN3 and recruits the PAN2-PAN3 complex to mRNA.
The CCR4-NOT complex hydrolyzes adenosine residues at the 3' end of polyadenylated mRNA, shortening the number of adenosine residues to about 10-15 residues and yielding adenosine 5'-monophosphate. CNOT6 and CNOT6L are the exoribonucleases responsible for hydrolysis. Activity of the CCR4-NOT complex is inhibited by PABP bound to the poly(A) tail of the mRNA. The trigger for activation of deadenylation by the CCR4-NOT complex is unknown. Complexes containing CNOT7 rather than CNOT8 appear to be responsible for cytoplasmic mRNA decay.
The PARN exoribonuclease hydrolyzes adenosine residues at the 3' ends of polyadenylated mRNA, shortening the poly(A) tail from about 80 adenosine residues to about 10-15 residues and yielding adenosine 5'-monophosphate. PARN interacts simultaneously with the poly(A) tail and with the 7-methylguanosine cap of the mRNA, therefore it is believed that PARN displaces the eIF4F cap-binding complex. The trigger for deadenylation by PARN is unknown. PARN is also part of a complex that regulates poly(A) tail length and hence translation in developing oocytes.
After deadenylation the mRNA is then hydrolyzed by either the 5' to 3' pathway or the 3' to 5' pathway. It is unknown what determinants target a mRNA to one pathway or the other.
The 5' to 3' pathway is initiated by binding of the Lsm1-7 complex to the 3' oligoadenylate tail followed by decapping by the DCP1-DCP2 complex. The 5' to 3' exoribonuclease XRN1 then hydrolyzes the remaining RNA.
The 3' to 5' pathway is initiated by the exosome complex at the 3' end of the mRNA. The exosome processively hydrolyzes the mRNA from 3' to 5', leaving only a capped oligoribonucleotide. The cap is then removed by the scavenging decapping enzyme DCPS.
Original Pathway at Reactome: http://www.reactome.org/PathwayBrowser/#DB=gk_current&FOCUS_SPECIES_ID=48887&FOCUS_PATHWAY_ID=429914
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