Nonsense-mediated mRNA decay is a surveillance system that rapidly degrades messenger RNAs containing premature stop codons. Nonsense -mediated mRNA decay has been observed in many eukaryotes including yeast, Drosophila, humans, and plants. This nonessential system of decay has been characterized genetically in C.elegans. Functions of the seven smg genes are required for nonsense-mediated mRNA decay. Mutations in any of these genes abolish NMD, and nonsense mutation-containing mRNAs accumulate to wild-type levels.
smg-2 encodes a protein containing a probable C6 zinc finger domain, and a type I RNA helicase domain with a nucleotide binding motif. Antibodies were raised against the amino terminal portion of a SMG-2, His-tagged, fusion protein. These antibodies detect, on a western blot, a single band of approximately 120 Kd in N2 animals. No signal is detected in
smg-2 null mutants. The same single 120Kd band is found in
smg-1,
smg-3,
smg-4, and
smg-6 mutants. In smg- 5 and
smg-7 mutant, however, a more slowly migrating SMG-2 isoform is present in addition to the abundant 120Kd isoform. SMG-5 and SMG-7 are noteworthy because they have been shown to form a complex (See abstract by Anders et al.). Expression of SMG-5, furthermore, is dependent on SMG-7 activity. Thus, SMG-5 is absent in both
smg-5 and
smg-7 mutants. We are currently testing whether activity of
smg-1, which encodes a putative PI-3/protein kinase (see abstract by O'Connor & Anderson), is necessary for accumulation of the more slowly migrating SMG-2 isoform. If it is, the slower SMG-2 isoform likely represents a phosphorylated derivative.