SMG-1 Is a Phosphatidylinositol Kinase-Related Protein Kinase Required for Nonsense-Mediated mRNA Decay in Caenorhabditis 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.

This work was supported by the University of Wisconsin Training Grant in Genetics, by a Howard Hughes Predoctoral Fellowship (to A.G.), and by a research grant (GM50933) from the NIH.

We thank Bob Barstead for the cDNA libraries, Sarah Crittenden for help with immunofluorescent microscopy, and Chris Malone for help with microparticle bombardment.