Analysis of the Drosophila Ajuba LIM protein defines functions for distinct LIM domains.

The Ajuba LIM protein Jub mediates regulation of Hippo signaling by cytoskeletal tension through interaction with the kinase Warts and participates in feedback regulation of junctional tension through regulation of the cytohesin Steppke. To investigate how Jub interacts with and regulates its distinct partners, we investigated the ability of Jub proteins missing different combinations of its three LIM domains to rescue jub phenotypes and to interact with α-catenin, Warts and Steppke. Multiple regions of Jub contribute to its ability to bind α-catenin and to localize to adherens junctions in Drosophila wing imaginal discs. Co-immunoprecipitation experiments in cultured cells identified a specific requirement for LIM2 for binding to Warts. However, in vivo, both LIM1 and LIM2, but not LIM3, were required for regulation of wing growth, Yorkie activity, and Warts localization. Conversely, LIM2 and LIM3, but not LIM1, were required for regulation of cell shape and Steppke localization in vivo, and for maximal Steppke binding in co-immunoprecipitation experiments. These observations identify distinct functions for the different LIM domains of Jub

Recruitment of Jub by α-catenin promotes Yki activity and Drosophila wing growth

International audienceThe Hippo signaling network controls organ growth through YAP family transcription factors, including the Drosophila Yorkie protein. YAP activity is responsive to both biochemical and biomechanical cues, with one key input being tension within the F-actin cytoskeleton. Several potential mechanisms for the biomechanical regulation of YAP proteins have been described, including tension-dependent recruitment of Ajuba family proteins, which inhibit kinases that inactivate YAP proteins, to adherens junctions. Here, we investigate the mechanism by which the Drosophila Ajuba family protein Jub is recruited to adherens junctions, and the contribution of this recruitment to the regulation of Yorkie. We identify α-catenin as the mechanotransducer responsible for tension-dependent recruitment of Jub by identifying a region of α-catenin that associates with Jub, and by identifying a region, which when deleted, allows constitutive, tension-independent recruitment of Jub. We also show that increased Jub recruitment to α-catenin is associated with increased Yorkie activity and wing growth, even in the absence of increased cytoskeletal tension. Our observations establish α-catenin as a multi-functional mechanotransducer and confirm Jub recruitment to α-catenin as a key contributor to biomechanical regulation of Hippo signaling