Worm Breeder's Gazette 14(3): 34 (June 1, 1996)
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.
Waksman Institute, Piscataway, NJ 08855
SMA-2, SMA-3, and SMA-4 are members of a family of cytoplasmic signaling molecules, that we call the dwarfins, which act downstream of TGF-b superfamily receptors. Other members of this family include the Drosophila Mad gene, three of the C.elegans daf genes (D. Riddle, J. Thomas, and G. Ruvkun, personal communications), and the human tumor suppressor gene DPC4. We have previously described that sma-2, sma-3, and sma-4 have very similar mutant phenotypes yet encode highly homologous gene products. These results have led to a model in which at least three dwarfin types are required together for signal transduction. If this model is correct, we are left with the question: why are multiple dwarfins required? More specifically, are there functional differences between these genes? Our ongoing characterization of these genes has begun to highlight some small differences between them. We have previously reported preliminary results on the expression pattern of sma-2. From several different sma-2-lacZ or -gfp constructs, we find that sma-2 is expressed nearly ubiquitously in larval stages, and in adults most highly in the pharynx, seam cells, and ventral hypodermis. We have now begun to look at sma-3-lacZ constructs. Surprisingly, the expression patterns are slightly different. Again, sma-3-lacZ is expressed very widely in larval stages, and shows significant staining in the adult pharynx. In contrast to sma-2, however, we see expression of sma-3 in intestinal cells in adults and larvae. In addition, we have seen expression of sma-3 in lateral and ventral hypodermal cells, but in many fewer animals and at much lower intensity than with sma-2. These similarities and differences in expression may allow us to begin to address whether and to what extent these two gene products act together in signal transduction. Since the staining of hypodermal cells with sma-3-lacZ is present in only a few animals, we will ask the question of whether sma-3 expression is temporally regulated, by looking more at staged animals and at sma-3-gfp expression in living animals. To address the question of functional redundancy, we had constructed the double and triple mutants sma-3 sma-2 and sma-4 sma-3 sma-2. We expected that if these genes were at least partially redundant, then the double and/or triple mutant would show a more severe phenotype. Instead, these mutant combinations showed no additional phenotypes: they are small and have male tail abnormalities, crumpled spicules and ray fusions. We concluded from this and other experiments that these genes are not functionally redundant. In characterizing these mutants more closely, however, we have noticed an unexpected result. The double mutant sma-3 sma-2 shows a smaller body size and increased penetrance of the male tail ray fusions than the single mutants. We think that this increase in severity is likely because the existing mutations are not nulls. The unexpected result is that, compared to the double mutant, the triple mutant sma-4 sma-3 sma-2 is not as small and has fewer ray fusions. So, even though sma-4 mutants have similar, not opposite, phenotypes to those of sma-2 and sma-3, when we construct the triple mutant, sma-4 acts like a suppressor. At present, we have no clear explanation for this phenomenon, but it suggests that we must consider models in which SMA-4 may have both positive and negative effects on signal transduction.