The mRNA Surveillance Protein hSMG-1 Functions in Genotoxic Stress Response Pathways in Mammalian Cells

Abstract

Members of the PI 3-kinase-related kinase (PIKK) family function in mitogenic and stress-induced signaling pathways in eukaryotic cells. Here, we characterize the newest PIKK family member, hSMG-1, as a genotoxic stress-activated protein kinase that displays some functional overlap with the related kinase, ATM, in human cells. Both ATM and hSMG-1 phosphorylate Ser/Thr-Gln-containing target sequences in the checkpoint protein p53 and the nonsense-mediated mRNA decay (NMD) protein hUpf1. Expression of hSMG-1 is required for optimal p53 activation after cellular exposure to genotoxic stress, and depletion of hSMG-1 leads to spontaneous DNA damage and increased sensitivity to ionizing radiation (IR). Moreover, IR exposure triggers hUpf1 phosphorylation at Ser/Thr-Gln motifs, and both ATM and hSMG-1 contribute to these phosphorylation events. Finally, NMD is suppressed in hSMG-1- but not ATM-deficient cells. These results indicate that hSMG-1 plays important roles in the maintenance of both genome and transcriptome integrity in human cells.

Identifying candidate colon cancer tumor suppressor genes using inhibition of nonsense-mediated mRNA decay in colon cancer cells

“…Sequential phosphorylation and dephosphorylation of the hUpf1 protein by the human suppressor with morphogenetic effect on genitalia-1 (hSMG-1) phosphatidylinositol 3-like kinase and protein phosphatase 2A (PP2A), respectively, is required for the initiation of NMD-induced mRNA degradation (Ohnishi et al, 2003;Brumbaugh et al, 2004). Moreover, a pharmacological block of NMD using caffeine or wortmannin, which inhibit hSMG-1 phosphorylation, has been reported previously (Usuki et al, 2004).…”

Phosphorylation of the Cryptochrome 1 C-terminal Tail Regulates Circadian Period Length

“…It has been proposed that one of the other PIKK family members can substitute for DNA-PK in DNA-PKcs null cells (60). Furthermore, most of the members of this family phosphorylate SQ motifs, and it has previously been reported that knockdown of one of these (SMG-1) can cause up-regulation of another (ATM), resulting in enhanced phosphorylation of the nonsense-mediated decay protein hUPF1 (61). Similarly, in our attempt to identify the kinase responsible for Ser-588 phosphorylation, we found that inhibition of ATM also caused an increase in Ser-588 phosphorylation (data not shown), suggesting that phosphorylation of this site may be regulated in a complex way, by more than one member of the PIKK family.…”

PNAS-4, an Early DNA Damage Response Gene, Induces S Phase Arrest and Apoptosis by Activating Checkpoint Kinases in Lung Cancer Cells

“…Recently, ATX, also known as SMG-1, has been found to be a member of the PIKK (phosphatidylinositol 3-kinase-related protein kinase) family, which includes ATM, ATR, TOR, SMG-1, DNA-PK, and TRRAP (40,61). ATX was first recognized for its role in regulating nonsense-mediated mRNA decay, an mRNA quality control pathway allowing for degradation of transcripts containing premature termination codons (39,62).…”

“…ATX was first recognized for its role in regulating nonsense-mediated mRNA decay, an mRNA quality control pathway allowing for degradation of transcripts containing premature termination codons (39,62). ATX was also shown to regulate cell cycle checkpoints in response to ionizing radiation and UV irradiation (40) and has been reported to be activated by UV light and in response to double-stranded breaks and subsequently to activate Chk1/2 (36). In the present study, we found that PNAS-4 did not cause activation of ATX (Fig.…”