SMG-1 Is a Phosphatidylinositol Kinase-Related Protein Kinase Required for Nonsense-Mediated mRNA Decay inCaenorhabditis elegans

Abstract

Eukaryotic messenger RNAs containing premature stop codons are selectively and rapidly degraded, a phenomenon termed nonsense-mediated mRNA decay (NMD). Previous studies with both Caenohabditis elegans and mammalian cells indicate that SMG-2/human UPF1, a central regulator of NMD, is phosphorylated in an SMG-1-dependent manner. We report here that smg-1, which is required for NMD in C. elegans, encodes a protein kinase of the phosphatidylinositol kinase superfamily of protein kinases. We identify null alleles of smg-1 and demonstrate that SMG-1 kinase activity is required in vivo for NMD and in vitro for SMG-2 phosphorylation. SMG-1 and SMG-2 coimmunoprecipitate from crude extracts, and this interaction is maintained in smg-3 and smg-4 mutants, both of which are required for SMG-2 phosphorylation in vivo and in vitro. SMG-2 is located diffusely through the cytoplasm, and its location is unaltered in mutants that disrupt the cycle of SMG-2 phosphorylation. We discuss the role of SMG-2 phosphorylation in NMD.Nonsense-mediated mRNA decay (NMD) rapidly and selectively degrades eukaryotic mRNAs containing premature translation termination codons (PTCs) (reviewed in references 27, 43, and 65). NMD likely improves the fidelity of gene expression by degrading aberrant mRNAs, thereby protecting cells from potentially deleterious consequences of their translation (9, 28, 53). The biological sources of NMD substrates are only partially understood. For example, unspliced, unproductively spliced, and aberrantly spliced mRNAs are degraded by NMD (29,39,48). NMD strongly influences the expression of certain genes for which mRNAs containing PTCs are a normal feature of their expression. For example, gene rearrangements of T-cell receptor genes often result in mRNAs that contain PTCs and are subjected to NMD (11).Gene products required for NMD have been identified in fungi, nematodes, insects, and mammals (13,16,24,49), and putative orthologs of these genes are evident in many sequenced eukaryotic genomes. A core group of three genes first identified in the yeast Saccharomyces cerevisiae (UPF1, NMD2/ UPF2, and UPF3) (reviewed in reference 69) are required for NMD in all tested eukaryotes (3,40,45,50,57,61). The conservation of both the sequence and function of such genes indicates that NMD is an evolutionarily ancient process and suggests that elements of the mechanism of NMD will be similar in most, or perhaps all, eukaryotes.PTCs are distinguished from normal termination codons in both yeast and mammalian cells by the context of translation termination. Specific cis-acting elements mark the open reading frame of mRNAs and activate NMD when translation terminates upstream of the marks. The Upf proteins appear to be involved both in sensing the presence of downstream signals and in activating the degradation machinery. In yeast cells, the cis-acting elements are termed downstream sequence elements, which are thought to be present throughout coding sequences but not in 3Ј untranslated regions (54). Hrp1p binds both downstream seq...

SPD-3 Is Required for Spindle Alignment in Caenorhabditis elegans Embryos and Localizes to Mitochondria

“…SMG-1 is a kinase that has been well studied in worms for its role in nonsense-mediated mRNA decay (NMD) (Grimson et al 2004). However, there is evidence that smg gene products may also be involved in other types of regulation of mRNA expression levels (Mango 2001;Mitrovich and Anderson 2005).…”

Smg1 is required for embryogenesis and regulates diverse genes via alternative splicing coupled to nonsense-mediated mRNA decay

“…Taken together with our results, a picture is emerging in which NMD factors play distinct developmental roles in different species. For example, in C. elegans, the Smg1 homologue is important for NMD but dispensable for survival (27). In D. melanogaster and D. rerio, the NMD factors Upf1 and Upf2, but not Smg1, are necessary for development (28,29).…”

Binding of a novel SMG-1–Upf1–eRF1–eRF3 complex (SURF) to the exon junction complex triggers Upf1 phosphorylation and nonsense-mediated mRNA decay

“…smg-1, smg-3, and smg-4 are required for the phosphorylation of SMG-2, whereas smg-5, smg-6, and smg-7 are required for dephosphorylation in C. elegans (Page et al 1999). Recent studies have also provided mechanistic insights into the regulation of Upf1/SMG-2 phosphorylation: SMG-1 kinase activity is essential for Upf1/SMG-2 phosphorylation (Yamashita et al 2001;Grimson et al 2004), whereas SMG-5 and SMG-7 are part of the protein phosphatase 2A (PP2A) complex Ohnishi et al 2003) and selectively associate with phosphorylated Upf1 in vivo (Ohnishi et al 2003). However, the relationship between the phosphorylation/dephosphorylation of Upf1 and the recognition of PTC and/or EJC by the surveillance complex remains to be clarified.…”