Characterization of human Smg5/7a: A protein with similarities to Caenorhabditis elegans SMG5 and SMG7 that functions in the dephosphorylation of Upf1

  1. SHANG-YI CHIU1,
  2. GUILLAUME SERIN1,3,
  3. OSAMU OHARA2, and
  4. LYNNE E. MAQUAT1
  1. 1Department of Biochemistry and Biophysics, School of Medicine and Dentistry, University of Rochester, Rochester, New York 14642, USA
  2. 2Department of Human Gene Research, Kazusa DNA Research Institute, Kisarazu, Chiba 292-0812, Japan; Immunogenomics Research Team, RIKEN Research Center for Allergy and Immunology, Yokohama, Japan

Abstract

Nonsense-mediated mRNA decay (NMD) in mammalian cells depends on phosphorylation of Upf1, an RNA-dependent ATPase and 5′-to-3′ helicase. Upf1 phosphorylation is mediated by Smg1, a phosphoinositol 3-kinase–related protein kinase. Here, we describe a human protein, which we call hSmg5/7a, that manifests similarity to Caenorhabditis elegans NMD factors CeSMG5 and CeSMG7, as well as two Drosophila melanogaster proteins that are also similar to the C. elegans NMD factors. Results indicate that hSmg5/7a functions in the dephosphorylation of Upf1. Furthermore, hSmg5/7a copurifies with Upf1, Upf2, Upf3X, Smg1, and the catalytic subunit of protein phosphatase 2A. We also demonstrate that Upf2, another factor involved in NMD, is a phosphoprotein. However, hSmg5/7a plays no role in the dephosphorylation of Upf2. These data indicate that hSmg5/7a targets protein phosphatase 2A to Upf1 but not Upf2. Results of Western blotting reveal that hSmg5/7a is mostly cytoplasmic in HEK293T cells.

Keywords

Footnotes

  • 3 Present address: Oncodesign, Parc Technologique de la Toison d’Or, 28 rue Louis de Broglie, 21000 Dijon, France.

  • Article and publication are at http://www.rnajournal.org/cgi/doi/10.1261/rna.2137903.

    • Accepted October 8, 2002.
    • Received September 10, 2002.
| Table of Contents