The post-translational modifier SUMO (small ubiquitin-like modifier) has a key role in regulation of transcription. The C. elegans SUMO,
smo-1 , was found in a synMuv screen, and may inhibit Ras signalling through interactions with chromatin-remodelling factors. The development of the C. elegans vulva provides an ideal model system for the identification of new interactors of SUMO. Ras signalling leads to adoption of the vulval cell fate by vulval precursor cells, and raised levels of this signal lead to the multivulva (Muv) phenotype. On this basis, we have performed a small-scale, candidate-based RNAi screen in a
smo-1 balanced background, in order to identify novel chromatin factor interactors of SUMO. We identified expected and novel interactors of
smo-1 , amongst which was the double bromodomain-encoding gene
bet-1 . Using
egl-17::cfp as a marker for high levels of Ras signalling in the vulval precursor cells, we found that animals depleted of
bet-1 show ectopic expression of
egl-17::cfp , suggesting that
bet-1 has a role in Ras signalling inhibition. As we suspected that loss of
bet-1 leads to hyperactivation of the Ras signalling cascade, we tested global levels of phosphorylated MPK-1 by western blotting. We found increased levels of phospho-MPK-1 in
bet-1 depleted animals. Since
bet-1 was identified in a
smo-1 synthetic screen, we constructed the double mutant
bet-1;
smo-1 , and observed an unexpected and striking synthetic Clear (synClr) phenotype. The Clr phenotype has been previously documented to be due to muscle degradation associated with elevated Ras signalling through the FGFR pathway. Taken together, this shows that
bet-1 plays an important role in the inhibition of Ras signalling in a range of tissues. As the human homolo gues of
bet-1 are implicated in epigenetic memory (due to their retention on mitotic chromosomes), further investigations could shed light upon the workings of this process.