Alternative titles; symbols
HGNC Approved Gene Symbol: SMG1
Cytogenetic location: 16p12.3 Genomic coordinates (GRCh38) : 16:18,804,860-18,926,408 (from NCBI)
Members of the phosphatidylinositol kinase (see 171834)-related kinase (PIKK) family, such as SMG1, play central roles in cell growth and stress response pathways. SMG1 is involved in nonsense-mediated mRNA decay, genotoxic and oxidative stress pathways, and TNF (191160)-induced apoptosis (summary by Yamashita et al., 2009).
In a yeast 2-hybrid screen of a human placenta cDNA library with the regulatory domain of lambda/iota PKC (PRKCI; 600539) as bait, Diaz-Meco et al. (1996) cloned SMG1, which they called LIP. The cDNA encodes a deduced 713-amino acid protein with a calculated molecular mass of 80 kD. Northern blot analysis revealed expression of a 6.5-kb transcript in placenta, brain, lung, and spleen, but not in liver.
By database searching for sequences showing homology to SMG1 of C. elegans, Denning et al. (2001) identified a partial sequence for human SMG1. They cloned a full-length cDNA from a chronic myelogenous leukemia cDNA library. The deduced 3,031-amino acid protein has a calculated molecular mass of 340 kD and contains a conserved ATP-binding site, a DXXXXN and a DXX motif within the catalytic domain, and a short FATC C-terminal region common in PIK-related kinases. Northern blot analysis detected wide expression of an 11.6-kb transcript in multiple tissues and cells, with highest expression in heart, skeletal muscle, kidney, and liver.
By successive backscreening, Yamashita et al. (2001) isolated overlapping cDNA clones of SMG1 and determined the full-length sequence. The deduced 3,657-amino acid protein has a calculated molecular mass of nearly 410 kD and shows about 50% identity with homologous proteins from C. elegans. A second putative start site encodes a 3,529-amino acid protein with a calculated molecular mass of 396 kD. Northern blot analysis detected expression of a 10-kb transcript in several human cell lines. In Western blot analysis of various cell lines of human, monkey, rat, and mouse origin, anti-SMG1 antibody recognized 2 proteins of 400 kD and 430 kD.
By searching databases for PI3-kinase-related kinases, followed by PCR of human brain and Jurkat T-cell cDNA libraries, Brumbaugh et al. (2004) cloned a SMG1 splice variant. The deduced 3,521-amino acid protein has a calculated molecular mass of 395 kD. This SMG1 protein is 8 amino acids shorter at the N terminus than the protein described by Yamashita et al. (2001) due to the inclusion of exon 4 instead of exon 5. Western blot analysis of transfected human embryonic kidney cells detected SMG1 at an apparent molecular mass of 395 kD.
With use of several recombinant PKC isotypes in in vitro binding assays, Diaz-Meco et al. (1996) confirmed specific interaction between LIP and the PRKCI isotype. By mutation analysis, they determined that LIP binds specifically to the zinc finger domain of PRKCI. With use of LIP-transfected HeLa cells, they found evidence for physical and functional interaction between LIP and PRKCI; LIP/PRKCI interaction could activate a reporter plasmid containing a kappa-B-dependent promoter.
Denning et al. (2001) found that immunoprecipitates of human kidney cells transfected with wildtype SMG1 were active in autophosphorylation assays and in assays using a general phospho-acceptor as substrate. SMG1 also exhibited a strong preference for Mn2+ over Mg2+ and was inhibited by nanomolar concentrations of wortmannin, similar to findings for other PIK-related kinases. It was not inhibited by rapamycin despite the presence of a putative rapamycin-binding domain.
Yamashita et al. (2001) found that both the 400-kD and 430-kD SMG1 proteins immunoprecipitated from HeLa cell lysates were capable of autophosphorylation. By mutation analysis, they determined that asp2331 is required for the intrinsic autophosphorylation activity. They also found that overexpression of SMG1, but not of a kinase-inactive point mutant, caused phosphorylation of specific serine residues in the C-terminal SQ motifs of UPF1/SMG2 (RENT1; 601430), a member of the mRNA surveillance complex. Using coexpression and immunoprecipitation assays, they determined that SMG1 associates with other components of the mRNA surveillance complex, UPF2 (605529) and UPF3A (605530). Wortmannin and caffeine were found to inhibit the kinase activity of SMG1, whereas staurosporine and rapamycin did not. Inhibitors of SMG1 induced the accumulation of truncated p53 in human cancer cell lines.
Brumbaugh et al. (2004) found that SMG1 was a genotoxic stress-activated protein kinase that displayed some functional overlap with ATM (607585) in human cells. Both ATM and SMG1 phosphorylated S/T-Q-containing target sequences in the checkpoint protein p53 (TP53; 191170) and UPF1. Expression of SMG1 was required for optimal p53 activation after cellular exposure to genotoxic stress, and depletion of SMG1 led to spontaneous DNA damage and increased sensitivity to ionizing radiation. Ionizing radiation exposure triggered UPF1 phosphorylation, and both ATM and SMG1 contributed to these phosphorylation events. Nonsense-mediated mRNA decay was suppressed in SMG1- but not ATM-deficient cells. Brumbaugh et al. (2004) concluded that SMG1 has a role in the maintenance of both genome and transcriptome integrity.
Grimson et al. (2004) identified null alleles of the C. elegans Smg1 gene and demonstrated that Smg1 kinase activity was required in vivo for nonsense-mediated mRNA decay (NMD) and in vitro for Smg2 phosphorylation.
SMG1 forms a surveillance complex with UPF1, ERF1 (ETF1; 600285), and ERF3 (GSPT1; 139259) on spliced mRNAs during recognition of premature termination codons. If an exon junction complex (EJC) exists downstream from the surveillance complex, SMG1 phosphorylates UPF1, and this step is rate limiting for NMD. Using reciprocal immunoprecipitation analysis, Yamashita et al. (2009) showed that SMG8 (613175) and SMG9 (613176) tightly associated with SMG1 in HeLa cell lysates, and that SMG9 appeared to mediate the interaction. Both SMG8 and SMG9 suppressed the kinase activity of SMG1, and both were SMG1 target substrates. Depletion of SMG9 resulted in accumulation of phosphorylated UPF1. SMG8 was required to recruit SMG1 to the surveillance complex, and depletion of SMG8 caused accumulation of a complex containing the ribosome, UPF1, ERF1, ERF3, and the EJC on the spliced mRNA-protein complex. Yamashita et al. (2009) concluded that their results provide a regulatory mechanism for SMG1 kinase activity, as well as a mechanism for premature termination codon recognition involving remodeling of the ribosome and surveillance complex.
Ishikawa et al. (1997) mapped the SMG1 gene to chromosome 16 by radiation hybrid analysis. By FISH, Yamashita et al. (2001) refined the localization to 16p12.
Brumbaugh, K. M., Otterness, D. M., Geisen, C., Oliveira, V., Brognard, J., Li, X., Lejeune, F., Tibbetts, R. S., Maquat, L. E., Abraham, R. T. The mRNA surveillance protein hSMG-1 functions in genotoxic stress response pathways in mammalian cells. Molec. Cell 14: 585-598, 2004. [PubMed: 15175154] [Full Text: https://doi.org/10.1016/j.molcel.2004.05.005]
Denning, G., Jamieson, L., Maquat, L. E., Thompson, E. A., Fields, A. P. Cloning of a novel phosphatidylinositol kinase-related kinase: characterization of the human SMG-1 RNA surveillance protein. J. Biol. Chem. 276: 22709-22714, 2001. [PubMed: 11331269] [Full Text: https://doi.org/10.1074/jbc.C100144200]
Diaz-Meco, M. T., Municio, M. M., Sanchez, P., Lozano, J., Moscat, J. Lambda-interacting protein, a novel protein that specifically interacts with the zinc finger domain of the atypical protein kinase C isotype lambda/iota and stimulates its kinase activity in vitro and in vivo. Molec. Cell. Biol. 16: 105-114, 1996. [PubMed: 8524286] [Full Text: https://doi.org/10.1128/MCB.16.1.105]
Grimson, A., O'Connor, S., Newman, C. L., Anderson, P. SMG-1 is a phosphatidylinositol kinase-related protein kinase required for nonsense-mediated mRNA decay in Caenorhabditis elegans. Molec. Cell. Biol. 24: 7483-7490, 2004. [PubMed: 15314158] [Full Text: https://doi.org/10.1128/MCB.24.17.7483-7490.2004]
Ishikawa, K., Nagase, T., Nakajima, D., Seki, N., Ohira, M., Miyajima, N., Tanaka, A., Kotani, H., Nomura, N., Ohara, O. Prediction of the coding sequences of unidentified human genes. VIII. 78 new cDNA clones from brain which code for large proteins in vitro. DNA Res. 4: 307-313, 1997. [PubMed: 9455477] [Full Text: https://doi.org/10.1093/dnares/4.5.307]
Yamashita, A., Izumi, N., Kashima, I., Ohnishi, T., Saari, B., Katsuhata, Y., Muramatsu, R., Morita, T., Iwamatsu, A., Hachiya, T., Kurata, R., Hirano, H., Anderson, P., Ohno, S. SMG-8 and SMG-9, two novel subunits of the SMG-1 complex, regulate remodeling of the mRNA surveillance complex during nonsense-mediated mRNA decay. Genes Dev. 23: 1091-1105, 2009. [PubMed: 19417104] [Full Text: https://doi.org/10.1101/gad.1767209]
Yamashita, A., Ohnishi, T., Kashima, I., Taya, Y., Ohno, S. Human SMG-1, a novel phosphatidylinositol 3-kinase-related protein kinase, associates with components of the mRNA surveillance complex and is involved in the regulation of nonsense-mediated mRNA decay. Genes Dev. 15: 2215-2228, 2001. [PubMed: 11544179] [Full Text: https://doi.org/10.1101/gad.913001]