AJUBA LIM Proteins Limit Hippo Activity in Proliferating Cells by Sequestering the Hippo Core Kinase Complex in the Cytosol.

The Hippo pathway controls organ growth and is implicated in cancer development. Whether and how Hippo pathway activity is limited to sustain or initiate cell growth when needed is not understood. The members of the AJUBA family of LIM proteins are negative regulators of the Hippo pathway. In mammalian epithelial cells, we found that AJUBA LIM proteins limit Hippo regulation of YAP, in proliferating cells only, by sequestering a cytosolic Hippo kinase complex in which LATS kinase is inhibited. At the plasma membranes of growth-arrested cells, AJUBA LIM proteins do not inhibit or associate with the Hippo kinase complex. The ability of AJUBA LIM proteins to inhibit YAP regulation by Hippo and to associate with the kinase complex directly correlate with their capacity to limit Hippo signaling during Drosophila wing development. AJUBA LIM proteins did not influence YAP activity in response to cell-extrinsic or cell-intrinsic mechanical signals. Thus, AJUBA LIM proteins limit Hippo pathway activity in contexts where cell proliferation is needed.

Ajuba LIM proteins are negative regulators of the Hippo signaling pathway.

The mammalian Ajuba LIM proteins (Ajuba, LIMD1, and WTIP) are adaptor proteins that exhibit the potential to communicate cell adhesive events with nuclear responses to remodel epithelia. Determining their role in vivo, however, has been challenging due to overlapping tissue expression and functional redundancy. Thus, we turned to Drosophila, where a single gene, CG11063 or djub, exists. Drosophila lacking the djub gene or depleted of dJub by RNA interference identify djub as an essential gene for development and a novel regulator of epithelial organ size as a component of the conserved Hippo (Hpo) pathway, which has been implicated in both tissue size control and cancer development. djub-deficient tissues were small and had decreased cell numbers as a result of increased apoptosis and decreased proliferation, due to downregulation of DIAP1 and cyclin E. This phenocopies tissues deficient for Yorkie (Yki), the downstream target of the Hippo pathway. djub genetically interacts with the Hippo pathway, and epistasis suggests that djub lies downstream of hpo. In mammalian and Drosophila cells, Ajuba LIM proteins/dJub interact with LATS/Warts (Wts) and WW45/Sav to inhibit phosphorylation of YAP/Yki. This work describes a novel role for the Ajuba LIM proteins as negative regulators of the Hippo signaling pathway.