title: Communication with the exon-junction complex and activation of nonsense-mediated decay by human Upf proteins occur in the cytoplasm.
abstract: The nonsense-mediated mRNA decay (NMD) pathway rids eukaryotic cells of mRNAs with premature termination codons. There is contradictory evidence as to whether mammalian NMD is a nuclear or a cytoplasmic process. Here, we show evidence that NMD in human cells occurs primarily, if not entirely, in the cytoplasm. Polypeptides designed to inhibit interactions between NMD factors specifically impede NMD when exogenously expressed in the cytoplasm. However, restricting the polypeptides to the nucleus strongly impairs their NMD-inhibitory function, even for those intended to inhibit interactions between the exon-junction complex (EJC) and hUpf3 proteins, which localize primarily in the nucleus. NMD substrates classified based on cell fractionation assays as "nucleus associated" or "cytoplasmic" are all inhibited in the same manner. Furthermore, retention of the NMD factor hUpf1 in the nucleus strongly impairs NMD. These observations suggest that the hUpf complex communicates with the EJC and triggers NMD in the cytoplasm.
authors.affiliation: Molecular, Cellular and Developmental Biology, University of Colorado, Boulder, CO 80309, USA.
authors.forename: Guramrit
authors.initials: G
authors.lastname: Singh
authors.forename: Steffen
authors.initials: S
authors.lastname: Jakob
authors.forename: Mark G
authors.initials: MG
authors.lastname: Kleedehn
authors.forename: Jens
authors.initials: J
authors.lastname: Lykke-Andersen
mesh_terms: D018389:Codon, Nonsense; D003593:Cytoplasm / Q000378:metabolism*; D005091:Exons; D006801:Humans; D008957:Models, Genetic; D009687:Nuclear Proteins / Q000737:chemistry / Q000378:metabolism; D017434:Protein Structure, Tertiary; D020365:RNA Helicases; D020871:RNA Stability / Q000502:physiology*; D012333:RNA, Messenger / Q000737:chemistry / Q000378:metabolism; D016601:RNA-Binding Proteins / Q000737:chemistry / Q000378:metabolism*; D015398:Signal Transduction; D015534:Trans-Activators / Q000737:chemistry / Q000378:metabolism*; D014157:Transcription Factors / Q000737:chemistry / Q000378:metabolism*