We have evidence that Ajuba plays a role in Rac dynamics, and conversely, Rac/PAK1 signalling modulates Ajuba-dependent junction stabilization. We will address in three Work Packages: 1 - how Ajuba modulates Rac dynamic. We postulate that Ajuba interaction may prevent Rac.GTP hydrolysis or facilitate recruitment of an exchange factor to junctions to activate Rac. Rac GTP loading and intrinsic hydrolysis will be addressed in the presence/absence of wild-type Ajuba or its phosphomimetic (T172D) or non-phosphorylatable (T172A) mutants. Ajuba mutants unable to interact with Rac will be made and tested in vivo for junction stabilization and rescue of Rac activation. As depletion of DOCK180, a Rac GEF, phenocopies Ajuba RNAi, a potential role for DOCK180 in Ajuba-dependent Rac activation by cadherins will be addressed. 2 - how Ajuba reorganizes actin at cadherin complexes. We will address the relative contribution of Ajuba-dependent actin polymerization versus bundling for cadherin stabilization and how Rac/PAK1 signalling influences these two activities. Mutational analysis will identify the F-actin binding sites on Ajuba responsible for polymerization or bundling and the efficiency of these mutants to strengthen cadherin adhesion. The existence of ternary complexes Rac/Ajuba/Actin or Rac/Ajuba/alpha-catenin will be studied as a mechanism to integrate cadherin complexes with actin cytoskeleton. 3 - the relevance of Ajuba in Rac/PAK1 signalling during tumour progression will be tested. Up-regulation of Ajuba protein and phosphorylation levels will be investigated in tumour tissue microarrays and cell lines (primary and metastatic site) and correlated with malignancy and clinical outcome. We will test Ajuba sub-cellular localization in tumours/cell lines (nucleus, junction or focal adhesion) and which cellular process it regulates downstream of Ras/Rac hyper-activation during transformation (proliferation, invasion and/or junction disassembly).