Mask proteins are cofactors of Yorkie/YAP in the Hippo pathway. Barry J Thompson, Clara M Sidor. London Research Institute, Cancer Research UK, London, United Kingdom.

   The Hippo signalling pathway acts via the Yorkie (Yki)/Yes-associated protein (YAP) transcriptional co-activator family to control tissue growth in both Drosophila and mammals. Yki/YAP drives tissue growth by activating target gene transcription, but how it does so remains unclear. Here we identify Mask as a novel co-factor for Yki/YAP. We show that Drosophila Mask forms a complex with Yki and its binding partner Scalloped on target gene promoters and is essential for Yki to drive transcription of target genes and tissue growth. Furthermore, the stability and sub-cellular localisation of both Mask and Yki is co-regulated in response to various stimuli. Finally, Mask proteins are functionally conserved between Drosophila and humans and are co-expressed with YAP in a wide variety of human stem/progenitor cells and tumours.