| Co-Investigator(Kenkyū-buntansha) |
HIRAI Syu-ichi Yokohama City thiiversity Graduate School of Medical Science, Department of Molecular Biology, Associate Professor, 医学研究科, 準教授 (80228759)
SUZUKI Atsushi Yokohama City University Graduate School of Medical Science, Department of Molecular Biology, Associate Professor, 医学研究科, 準教授 (00264606)
MIZUNO Keiko Yokohama City University School of Medicine, Department of Molecular Biology, Assistant Professor, 医学部, 助手 (90221803)
AKIMOTO Kazunori Yokohama City University School of Medicine, Department of Molecular Biology, Assistant Professor, 医学部, 助手 (70285104)
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| Research Abstract |
Nonsense-mediated mRNA decay (NMD) is a surveillance mechanism that degrades mRNA containing premature termination codons (PTCs). In mammalian cells, recognition of PTCs requires translation and depends on the presence on the mRNA with the splicing-dependent exon junction complex (EJC). While it is known that a key event in the triggering of NMD is phosphorylation of the trans-acting factor, Upf1, by SMG-1, the relationship between Upf1 phosphorylation and PTC recognition remains undetermined. Here we show that SMG-1 binds to the mRNA-associated components of the EJC, Upf2, Upf3b, elF4A3, Magoh, and Y14. Further, we describe a novel complex that contains the NMD factors SMG-1 and Upf1, and the translation termination release factors eRF1 and eRF3 (SURF). Importantly, an association between SURF and the EJC is required for SMG-1-mediated Upf1 phosphorylation and NMD. Thus, the SMG-1-mediated phosphorylation of Upf1 occurs on the association of SURF with EJC, which provides the link betw
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een the EJC and recognition of PTCs and triggers NMD.
Phosphatidylinositol 3-kinase-related kinases (PIKKs) consisting of SMG-1, ATM, ATR, DNA-PKcs, and mTOR are a family of proteins involved in the surveillance of gene expression ineukaryotic cells. They are involved in mechanisms responsible for genome stability, mRNA quality, and translation. They share a large N-terminal domain and a C-terminal FATC domain in addition tothe unique serine/threonine protein kinase (PIKK) domain that is different from classical proteinkinases. However, structure-function relationships of PIKKs remain unclear. Here we have focused onone of the PIKK members, SMG-1 that is involved in RNA surveillance termed nonsense-mediatedmRNA decay (N MD), to analyze the roles of conserved and SMG-1 specific sequences on the intrinsickinase activity. Analyses of sets of point and deletion mutants of SMG-1 in a purified system and intactcells revealed that the long N-terminal region and the conserved leucine in the FATC domain wereessential for SMG-1 kinase activity. However, the conserved tryptophan in the TS domain and theFATC domain was not. In addition, the long insertion region between PIKK and FATC domains wasnot essential for SMG-1 kinase activity. These results indicated an unexpected feature of SMG-1, i.e.,the distantly located N-terminal and C-terminal sequences were essential for the intrinsic kinaseactivity.
In order to confirm our previous results using SMG-1 inhibitors, we evaluated the effects of NMD inhibition by siRNA-mediated knockdown of SMG-1 or Upf1 on the phenotype of Ullrich disease, an autosomal recessive congenital muscular dystrophy. The patient studied showed a homozygousframeshift mutation with a PTC in the collagen VI A2 gene, which encodes a truncated but partiallyfunctional protein. The patient's fibroblasts showed a nearly complete loss of the triple-helical collagenVI protein and functional defects in the extracellular matrix (ECM) due to the crucial deficiency of the collagen VI a2 protein. We have shown that siRNA-mediated knockdown of SMG-1 or Upf1 causes theupregulation of the mutant triple-helical collagen VI, resulting in the formation of partially functionalECM. We suggest that the inhibition of NMD may be useful as a therapeutic approach to treat somehuman genetic diseases exacerbated by NMD. Less
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