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Molecular and Cellular Biology, August 2007, p. 5630-5638, Vol. 27, No. 16
0270-7306/07/$08.00+0     doi:10.1128/MCB.00410-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Caenorhabditis elegans SMG-2 Selectively Marks mRNAs Containing Premature Translation Termination Codons ^,

Lisa Johns,^1,2, Andrew Grimson,^1,, Sherry L. Kuchma,^1,¶ Carrie Loushin Newman,^1,|| and Philip Anderson^1,2*

Department of Genetics,¹ Program in Cellular and Molecular Biology, University of Wisconsin, Madison, Wisconsin 53706²

Received 8 March 2007/ Returned for modification 27 March 2007/ Accepted 4 June 2007

Eukaryotic mRNAs containing premature translation termination codons (PTCs) are rapidly degraded by a process termed "nonsense-mediated mRNA decay" (NMD). We examined protein-protein and protein-RNA interactions among Caenorhabditis elegans proteins required for NMD. SMG-2, SMG-3, and SMG-4 are orthologs of yeast (Saccharomyces cerevisiae) and mammalian Upf1, Upf2, and Upf3, respectively. A combination of immunoprecipitation and yeast two-hybrid experiments indicated that SMG-2 interacts with SMG-3, SMG-3 interacts with SMG-4, and SMG-2 interacts indirectly with SMG-4 via shared interactions with SMG-3. Such interactions are similar to those observed in yeast and mammalian cells. SMG-2-SMG-3-SMG-4 interactions require neither SMG-2 phosphorylation, which is abolished in smg-1 mutants, nor SMG-2 dephosphorylation, which is reduced or eliminated in smg-5 mutants. SMG-2 preferentially associates with PTC-containing mRNAs. We monitored the association of SMG-2, SMG-3, and SMG-4 with mRNAs of five endogenous genes whose mRNAs are alternatively spliced to either contain or not contain PTCs. SMG-2 associates with both PTC-free and PTC-containing mRNPs, but it strongly and preferentially associates with ("marks") those containing PTCs. SMG-2 marking of PTC-mRNPs is enhanced by SMG-3 and SMG-4, but SMG-3 and SMG-4 are not detectably associated with the same mRNPs. Neither SMG-2 phosphorylation nor dephosphorylation is required for selective association of SMG-2 with PTC-containing mRNPs, indicating that SMG-2 is phosphorylated only after premature terminations have been discriminated from normal terminations. We discuss these observations with regard to the functions of SMG-2 and its phosphorylation during NMD.

Published ahead of print on 11 June 2007.

Supplemental material for this article may be found at http://mcb.asm.org/.

L.J. and A.G. contributed equally to this work.

Present address: Whitehead Institute, 9 Cambridge Center, Cambridge, MA, 02142.

^¶ Present address: Department of Microbiology and Immunology, Dartmouth Medical School, Hanover, NH 03755.

^|| Present address: Department of Biochemistry, University of Wisconsin, Madison, WI 53706.

This article has been cited by other articles:

• Johansson, M. J. O., He, F., Spatrick, P., Li, C., Jacobson, A. (2007). Association of yeast Upf1p with direct substrates of the NMD pathway. Proc. Natl. Acad. Sci. USA 104: 20872-20877 [Abstract] [Full Text]