A non-transcriptional role for Hippo pathway signaling. Pam Vanderzalm, Richard Fehon. Molecular Genetics & Cell Biology, University of Chicago, Chicago, IL.

   The Hippo-Salvador-Warts (HSW) tumor suppressor pathway has been well-characterized with respect to its ability to regulate growth through Yorkie-dependent transcription. Genes involved in promoting growth (cyclin E and bantam miRNA) or inhibiting cell death (diap1), as well as upstream activators of the pathway (expanded and kibra), are bone fide transcriptional outputs of the pathway.
    In addition to regulating growth, the HSW pathway also regulates the polarity of epithelial cells by limiting the size of the adherens junction (AJ) and the apical domain (AD). Cells either lacking HSW function or overexpressing Yorkie have higher levels of many proteins that localize to the AD, including those involved in regulating apical polarity (such as Crumbs and aPKC). AJ proteins such as Armadillo and E-Cadherin are similarly upregulated. Many, if not all, of the components of the HSW pathway localize to the AD, and localization of HSW components may be critical for activating the pathway. For instance, tethering Mats to the membrane activates signaling through the HSW pathway.
    We examined whether the apical localization of Yorkie was important for its role in controlling apical domain size. We also asked whether transcription through Yorkie was required to control AJ and AD size. By generating transgenic flies expressing a version of Yorkie that is transcriptionally-dead, we found that Yorkie, and by extension HSW signaling, controls the levels of apical polarity proteins at the membrane independent of its function in growth-regulating transcription. Consistent with this finding, mutating Ser168 to alanine, which promotes Yorkie translocation to the nucleus, diminishes Yorkies ability to promote apical identity. Detailed characterization of the polarity phenotype and an analysis of HSW complex formation at the apical membrane will be presented.