Characterization of YAN, a Negative Component of Receptor Tyrosine Kinase Signaling. T. Tootle , I. Rebay. MIT, Whitehead Institute, Cambridge, MA.

   Yan, a transcriptional repressor belonging to the ETS transcription factor family, plays an important role in controlling differentiation and proliferation of many cell types. In the context of RAS/MAPK signaling, this transcriptional repression is removed by MAPK phosphorylation of YAN, thereby allowing differentiation to proceed. Our model for YAN down regulation is that phosphorylated YAN must be exported from the nucleus and subsequently degraded in the cytoplasm. We will present the results of genetic and biochemical experiments designed to explore the mechanisms of YAN nuclear export and degradation. First, structure/function analyses will be used to define the domains of YAN that regulate its subcellular localization. Second, we are investigating whether Yan nuclear export is sensitive to the drug Leptomycin B, a specific inhibitor of CRM-1 mediated export. Third, we are examining whether YAN activity and localization is similarly regulated by other MAPK family members including JNK, p38a and p38b. Finally, we will present the results of a genetic screen designed to look for mutations in which YAN is either mislocalized or not properly down-regulated. Mutations affecting degradation are expected to show increased cytoplasmic accumulation of YAN, whereas mutations affecting export should retain nuclear YAN expression in cells that would normally downregulate YAN prior to differentiation.