Worm Breeder's Gazette 14(1): 90 (October 1, 1995)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Laboratory of Genetics University of Wisconsin-Madison
Nonsense mutant mRNAs are generally less stable than WT mRNAs. The seven smg genes are required for this "nonsense-mediated" mRNA decay. In smg mutants, otherwise unstable nonsense mRNAs accumulate to approximately WT levels. We have previously reported the cloning of smg-5 (Anders and Anderson, 1995 Worm Meeting Abstracts) and smg-7 (Cali and Anderson, 1995 Worm Meeting Abstracts). smg-5 encodes a 549 amino acid protein that is not similar to any protein sequence in the public databases. smg-7 is predicted to encode a 53 kDa protein with two notable features: (i.) three repeats of a conserved 34 amino acid domain known as a tetratricopeptide repeat (TPR) and (ii.) a negatively charged COOH-terminus. TPRs are predicted to form amphipathic helices and are believed to mediate homo- and heteromeric protein:protein interactions. Here we describe the characterization of SMG-5- and SMG-7-specific antisera and provide evidence that these proteins associate in vivo. Polyclonal antisera to both SMG-5 and SMG-7 were generated in rabbits using bacterially expressed fusion proteins as immunogen. Both antisera specifically detect 0.1 ng of the appropriate fusion protein. Antisera raised to SMG-5 fusion protein detect a 56 kDa protein in N2 but not smg-5(-) extracts by immunoprecipitation followed by Western blotting. Antisera raised to SMG-7 fusion protein detect 54 and 57 kDa proteins on Western blots of total worm protein. Several results indicate the 54 kDa protein is encoded by smg-7. First, its apparent molecular weight is quite close to that predicted for SMG-7. Second, the abundance of the 54 kDa protein is reduced (but not eliminated) in smg-7(r1131::Tc1) animals. smg-7(r1131::Tc1) animals accumulate approximately 60% of WT levels of WT-sized smg-7 mRNA. Thus, it is not surprising that this mutation does not eliminate SMG-7 synthesis. We are currently investigating whether the 57 kDa protein is also encoded by smg-7 (perhaps by an alternatively spliced smg-7 mRNA), and are considering the possibility this protein represents a post-translationally modified form of SMG-7. The presence of a protein:protein interaction domain in SMG-7 suggests it might associate with other proteins, perhaps including the products of some or all of the other smg genes. Therefore, we have tested whether SMG-5 and SMG-7 associate in vivo as judged by co-immunoprecipitation. Both SMG-5 and SMG-7 co-precipitate from extracts when either antisera is used. The proteins are detected by Western blot analysis of immunoprecipitates. Coprecipitation of SMG-7 with SMG-5-specific antisera requires WT SMG-5, as SMG-7 does not coprecipitate from smg-5(-) extracts (although SMG-7 is present in smg-5(-) extracts). Only the 54 kDa (not the 57 kDa) polypeptide identified by anti-SMG-7 antisera co-precipitates with SMG-5. We conclude that SMG-5 and SMG-7 physically interact as components of a multi-protein complex. Work is in progress to determine if the two proteins interact directly or indirectly, and if the integrity of the complex is maintained in other smg mutants.