Abstract
Hippo signalling integrates diverse stimuli related to epithelial architecture to regulate tissue growth and cell fate decisions. The Hippo kinase cascade represses the growth-promoting transcription co-activator Yorkie. The FERM protein Expanded is one of the main upstream Hippo signalling regulators in Drosophila as it promotes Hippo kinase signalling and directly inhibits Yorkie. To fulfil its function, Expanded is recruited to the plasma membrane by the polarity protein Crumbs. However, Crumbs-mediated recruitment also promotes Expanded turnover via a phosphodegron-mediated interaction with a Slimb/β-TrCP SCF E3 ligase complex. Here, we show that the Casein Kinase 1 (CKI) family is required for Expanded phosphorylation. CKI expression promotes Expanded phosphorylation and interaction with Slimb/β-TrCP. Conversely, CKI depletion in S2 cells impairs Expanded degradation downstream of Crumbs. In wing imaginal discs, CKI loss leads to elevated Expanded and Crumbs levels. Thus, phospho-dependent Expanded turnover ensures a tight coupling of Hippo pathway activity to epithelial architecture.
Highlights
The maintenance of epithelial tissue architecture through cell-cell and cell-extracellular matrix contacts, as well as cell polarity, is essential for organ function and size control (Genevet and Tapon, 2011; Low et al, 2014)
The Ex:Slimb (Slmb, Drosophila b-TrCP) interaction is mediated by a b-TrCP consensus sequence immediately following the Ex N-terminal FERM domain (452TSGIVS457)
We used affinity purification coupled with mass spectrometry (AP-MS) to identify Slmb as an Ex interacting protein (Ribeiro et al, 2014)
Summary
The maintenance of epithelial tissue architecture through cell-cell and cell-extracellular matrix contacts, as well as cell polarity, is essential for organ function and size control (Genevet and Tapon, 2011; Low et al, 2014). When Hpo signalling is inactive, Yki/YAP is able to enter the nucleus, associate with its transcription factor partner Scalloped (TEAD1-4 in mammals) and promote the expression of cell cycle regulators and apoptosis inhibitors, among others (Genevet and Tapon, 2011; Yu and Guan, 2013; Hong and Guan, 2012). To ensure that epithelial homeostasis is maintained, Yki/YAP control the expression of Hpo pathway upstream regulators that dampen Yki/YAP activity as part of a negative feedback mechanism (Genevet and Tapon, 2011; Halder and Johnson, 2011).
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