Search results for SMG8

Showing 6 results out of 6

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Species

Types

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Protein (1 results from a total of 1)

Identifier: R-HSA-927759
Species: Homo sapiens
Compartment: cytosol
Primary external reference: UniProt: SMG8: Q8ND04

Complex (1 results from a total of 1)

Identifier: R-HSA-927853
Species: Homo sapiens
Compartment: cytosol

Reaction (2 results from a total of 2)

Identifier: R-HSA-927889
Species: Homo sapiens
Compartment: cytosol
SMG1 phosphorylates UPF1 in vitro and in vivo (Denning et al. 2001, Yamashita et al. 2001, Kashima et al. 2006). Serines 1073, 1078, 1096, and 1116 in isoform 2 (Serines 1084, 1089, 1107, 1127 in isoform 1) are phosphorylated in vitro and phosphorylation at serines 1078 and 1096 has been confirmed in vivo (Yamashita et al. 2001, Ohnishi et al. 2003, Kashima et al. 2006). UPF1 also contains additional serine and threonine residues that could be phosphorylated. SMG8 and SMG9 associate with SMG1 and regulate the kinase activity of SMG1 (Yamashita et al. 2009). The phosphorylation reaction is rate-limiting in nonsense-mediated decay and is therefore regarded as a licensing step (reviewed in Rebbapragada and Lykke-Andersen 2009). Phosphorylation is enhanced by the exon junction complex, which can interact with UPF1 via UPF2 and/or UPF3 (Kashima et al. 2006, Ivanov et al. 2008) or via Y14:Magoh (Ivanov et al. 2008). SMG8 and SMG9 bind SMG1 and regulate its kinase activity (Yamashita et al. 2009, Fernandez et al. 2011).
Identifier: R-HSA-927832
Species: Homo sapiens
Compartment: cytosol
The presence of an exon junction complex (EJC) downstream of a termination codon enhances nonsense-mediated decay (NMD) but is not absolutely required for NMD. The EJC is deposited during splicing and remains bound to the mRNA until a ribosome dislodges it during the pioneer round of translation, distinguished by the presence of the cap-binding complex at the 5' end. If translation terminates at least 50-55 nucleotides 5' to an EJC during the pioneer round then termination factors (eRF1 and eRF3) and the EJC recruit UPF1 and other NMD machinery (Lykke-Andersen et al. 2001, Ishigaki et al. 2001, Le Hir et al. 2001, Gehring et al. 2003, Hosoda et al. 2005, Kashima et al. 2006, Singh et al. 2007, Chamieh et al. 2008, Ivanov et al. 2008, Buchwald et al. 2010).
A current model for NMD enhanced by the EJC posits recruitment of UPF1, SMG1, SMG8, and SMG9 to eRF3 at the ribosome to form the SURF complex (Kashima et al. 2006, Chang et al. 2007, Isken et al. 2008, Muhlemann et al. 2008, Stalder and Muhlemann 2008, Chamieh et al. 2009, Maquat and Gong 2009, Rebbapragada and Lykke-Andersen 2009, Hwang et al. 2010, Nicholson et al. 2010). UPF1 and SMG1 then interact with components of the EJC, activating phosphorylation of UPF1 by SMG1.
The model of the NMD mechanism is inferred from known protein interactions:
eRF1 and eRF3 interact with UPF1, the key regulator of NMD which also binds SMG1, UPF2, and UPF3 (UPF3a or UPF3b) to form the SURF complex (Kashima et al.2006, Ivanov et al. 2008, Clerici et al. 2009, Chakrabarti et al. 2011). UPF1 also interacts with CBP80 at the cap of the mRNA (Hwang et al. 2010).
SMG8 and SMG9 associate with SMG1 and the SURF complex and modulate the phosphorylation activity of SMG1 (Yamashita et al. 2009).
UPF2 and UPF3 are peripheral components of the EJC and thus may link the EJC to the SURF complex (Chamieh et al. 2008). UPF3b binds UPF1 and a composite surface formed by the Y14, MAGOH, and eIF4A3 subunits of the core EJC (Gehring et al. 2003, Kunz et al. 2006, Buchwald et al. 2010). SMG1 also interacts with the EJC (Kashima et al. 2006, Yamashita et al. 2009). UPF3a more weakly activates NMD than does UPF3b (Kunz et al. 2006) and UPF3a levels increase in response to loss of UPF3b (Chan et al. 2009).
The binding of UPF1 to translated RNAs may occur in two steps: Binding of the SURF complex to the terminating ribosome followed by transfer of UPF1 and SMG1 to the EJC (Kashima et al. 2006, Hwang et al. 2010).
The core EJC (Y14, MAGOH, eIF4A3, and BTZ) can activate NMD without UPF2, however RNPS1, another EJC subunit, requires UPF2 to activate NMD (Gehring et al. 2005). RNAs show differential dependence on RNPS1-activated NMD (Gehring et al. 2005). Also, NMD of some transcripts requires EJC component eIF4A3 but not UPF3b (Chan et al. 2007) therefore there may be more than one route to activating NMD via the EJC.

Pathway (2 results from a total of 2)

Identifier: R-HSA-975957
Species: Homo sapiens
Compartment: cytosol
During normal translation termination eRF3 associates with the ribosome and then interacts with PABP bound to the polyadenylate tail of the mRNA to release the ribosome and allow a new round of translation to commence. Nonsense-mediated decay (NMD) is triggered if eRF3 at the ribosome interacts with UPF1, which may compete with PABP (reviewed in Isken and Maquat 2007, Chang et al. 2007, Behm-Ansmant et al. 2007, Rebbapragada and Lykke-Andersen 2009, Bhuvanagiri et al. 2010, Nicholson et al. 2010, Durand and Lykke-Andersen 2011). An exon junction located 50-55 nt downstream of a termination codon is observed to enhance NMD.
Exon-junction complexes (EJCs) are deposited on the mRNA during splicing in the nucleus, remain on mRNAs after transport to the cytosol, and are dislodged by the ribosome as it progresses along the mRNA during the pioneer round of translation (Gehring et al. 2009). EJCs contain the core factors eIF4A-III, Magoh-Y14, and CASC3 as well as the peripheral factors RNPS1, UPF2, and UPF3. UPF2 and UPF3 recruit UPF1 to eRF3 at the terminating ribosome. Thus an EJC downstream of a termination codon will not have been dislodged during translation and will recruit UPF1, triggering NMD.
UPF1 is believed to form a complex containing SMG1, SMG8, and SMG9. In the key regulatory step of NMD SMG1 phosphorylates UPF1. The phosphorylated UPF1 then recruits either SMG6 or SMG5 and SMG7. SMG6 is itself an endoribonuclease that cleaves the mRNA. SMG5 and SMG7 do not have endoribonuclease activty, but are thought to recruit ribonucleases. Nonsense-mediated decay has been observed to involve deadenlyation, decapping, and both 5' to 3' and 3' to 5' exonuclease activities, but the exact degradative pathways taken by a given mRNA are not yet known.
UPF1 also plays roles in Staufen-mediated decay, histone mRNA decay, telomere maintenance, genome integrity, and may play a role in normal termination of translation.
Identifier: R-HSA-927802
Species: Homo sapiens
Compartment: cytosol
The Nonsense-Mediated Decay (NMD) pathway activates the destruction of mRNAs containing premature termination codons (PTCs) (reviewed in Isken and Maquat 2007, Chang et al. 2007, Behm-Ansmant et al. 2007, Neu-Yilik and Kulozik 2008, Rebbapragada and Lykke-Andersen 2009, Bhuvanagiri et al. 2010, Nicholson et al. 2010, Durand and Lykke-Andersen 2011). In mammalian cells a termination codon can be recognized as premature if it precedes an exon-exon junction by at least 50-55 nucleotides or if it is followed by an abnormal 3' untranslated region (UTR). While length of the UTR may play a part, the qualifications for being "abnormal" have not been fully elucidated. Also, some termination codons preceding exon junctions are not degraded by NMD so the criteria for triggering NMD are not yet fully known (reviewed in Rebbapragada and Lykke-Andersen 2009). While about 30% of disease-associated mutations in humans activate NMD, about 10% of normal human transcripts are also degraded by NMD (reviewed in Stalder and Muhlemann 2008, Neu-Yilik and Kulozik 2008, Bhuvanagiri et al. 2010, Nicholson et al. 2010). Thus NMD is a normal physiological process controlling mRNA stability in unmutated cells.
Exon junction complexes (EJCs) are deposited on an mRNA during splicing in the nucleus and are displaced by ribosomes during the first round of translation. When a ribosome terminates translation the A site encounters the termination codon and the eRF1 factor enters the empty A site and recruits eRF3. Normally, eRF1 cleaves the translated polypeptide from the tRNA in the P site and eRF3 interacts with Polyadenylate-binding protein (PABP) bound to the polyadenylated tail of the mRNA.
During activation of NMD eRF3 interacts with UPF1 which is contained in a complex with SMG1, SMG8, and SMG9. NMD can arbitrarily be divided into EJC-enhanced and EJC-independent pathways. In EJC-enhanced NMD, an exon junction is located downstream of the PTC and the EJC remains on the mRNA after termination of the pioneer round of translation. The core EJC is associated with UPF2 and UPF3, which interact with UPF1 and stimulate NMD. Once bound near the PTC, UPF1 is phosphorylated by SMG1. The phosphorylation is the rate-limiting step in NMD and causes UPF1 to recruit either SMG6, which is an endoribonuclease, or SMG5 and SMG7, which recruit ribonucleases. SMG6 and SMG5:SMG7 recruit phosphatase PP2A to dephosphorylate UPF1 and allow further rounds of degradation. How EJC-independent NMD is activated remains enigmatic but may involve competition between PABP and UPF1 for eRF3.
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