Characterizing the interaction between dCAF1-p180 and the tumor suppressor Merlin. Patrick Delaney, Pam Vanderzalm, Richard Fehon. Molecular Genetics and Cell Biology, University of Chicago, Chicago, IL.

   Merlin (Mer), the product of the Neurofibromatosis 2 tumor suppressor gene, acts upstream in the highly conserved Hippo (Hpo) tumor suppressor pathway to regulate cell proliferation and apoptosis. While Mer plays an active role in upstream Hpo signaling, the mechanism by which Mer itself is regulated remains poorly characterized. To study this regulation, a yeast 2-hybrid (Y2H) screen and subsequent functional assays were carried out to identify proteins that interact with Mer in vivo. We identified dCAF1-p180 (p180), a member of the Chromatin Assembly Factor 1 complex, as a protein that interacts with Mer. Consistent with the Y2H results, p180 can co-immunoprecipitate with wild-type and active variants of Mer when expressed in Drosophila S2 cells. p180 is primarily cytoplasmic and membrane-associated when expressed in S2 cells, but epitope-tagged p180 appears primarily nuclear in imaginal epithelial cells. RNAi depletion of p180 in wing imaginal discs results in a complex growth phenotype with increased levels of Mer. However, we observe decreased levels of the adherens junction protein Ecadherin, and other readouts of Hpo signaling, such as Expanded, are not affected by loss of p180 function. Together these results suggest that p180 affects Merlin expression but does not function directly in the Hpo pathway.
   p180 plays important nuclear roles in DNA damage repair and maintenance of epigenetic memory. In contrast, Mer is thought to regulate signaling at the cell membrane due to its subcellular localization and interactions with transmembrane proteins. However, recent evidence in mammals suggests Mer may also have an important nuclear role. Given our results, we speculate that Mer may repress its own transcription via interactions with p180. Further elucidation of this interaction could reveal another layer of control for Mer and Hpo signaling, and would expand our understanding of the recently discovered class of chromatin modifying tumor suppressor genes.