Brandon Weasner, Research Associate
Members of the SIX family of DNA binding proteins play key roles in the development of a myriad of tissues including the retina in both flies and vertebrates. In Drosophila this family is represented by the sine oculis, optix and DSix4 genes. In flies, sine oculis and optix are expressed in dynamic patterns within the developing eye with both genes being expressed ahead of the morphogenetic furrow but with only sine oculis being transcribed in developing photoreceptor neurons behind the furrow. The loss of sine oculis leads to a derailment of the eye specification program, a dramatic drop in cell proliferation rates and a steep rise in apoptosis levels. This ultimately results in adult flies that completely lack the compound eyes.
In addition to the homeobox, each of these three proteins contains an evolutionarily conserved protein-protein interaction motif (called the SIX domain), which mediates interactions with the Eyes absent (Eya) family of transcriptional co-activators and/or the Groucho (Gro) family of co-repressors. We, and others, have shown that the ability of Sine oculis to both activate and repress the transcription of target genes is essential for proper eye development.
However, there is some dispute as to whether all SIX proteins can serve as both transcriptional activators and repressors. It is also unclear if co-factors like Eya and Gro compete for binding to the same sites within the SIX proteins. I am taking a biochemical approach to solving these problems and am in the process of using co-immunoprecipitations from transfected S2 cells to determine the binding preferences for all Drosophila and mouse SIX proteins. I am also determining the exact location within the SIX domain that is bound by known co-activators and co-repressors. The results of these experiments will be important as they are likely to provide considerable insight into the mechanisms by which all SIX proteins regulate transcription and influence development.
