cascade regulating cell contact inhibition, organ size control, cell growth, proliferation, apoptosis and cancer development in mammals. The D. melanogaster protein Expanded acts in the Hippo signaling pathway to control organ size. Previously, willin/FRMD6 has been proposed as the human orthologue of Expanded. Willin lacks C-terminal sequences that are present in Expanded and, to date, little is known about the functional role of willin in mammalian cells. When willin is expressed in D. melanogaster epithelial
tissues, it has the same sub-cellular localization as Expanded, but cannot rescue growth defects associated with expanded deficiency. However, we show that ectopic willin expression causes an increase in phosphorylation of the core Hippo signaling pathway components MST1/2, LATS1 and YAP, an effect which can be antagonized by ezrin. In MCF10A cells, loss of willin expression displays epithelial-to-mesenchymal transition features and willin over-expression antagonizes YAP activity via the N-terminal FERM domain of willin. Therefore, in mammalian cells willin influences Hippo signaling activity by activating the core Hippo pathway kinase cassette.