Integrins are transmembrane receptors composed of ? and ? subunits that mediate the interactions between cells and ECM. In mammals, there are 18 ? and 8 ? subunits, which combine to form dimers that exhibit different ligand binding properties. Integrins are classified by these ligand binding properties into collagen, LM and RGD binding receptors. The principal LM binding integrins are ?3?1, ?6?1 and ?6?4. While specificity to ligand is mediated by the integrin extracellular domains, their transmembrane domain (TM) and cytoplasmic tail (CT) play a critical role in mediating activation of selective intracellular signaling, thus contributing to specificity of integrin function. Integrin-mediated cell interactins with LMs are critical for renal collecting system development. We previously showed that deleting the integrin ?3 subunit in the UB in mice gives a developmental phenotype and, similar to the recent findings in humans, makes mice susceptible to renal fibrosis after unilateral ureteric obstruction (UUO). By contrast, deleting the integrin ?6 or ?4 subunits did not alter UB development; but UUO caused severe tubular dilatation. Our preliminary data show that integrin ?3/?6 compound mutant mice develop significantly worse renal phenotypes than single mutants, suggesting that cooperation between integrins ?3?1 and ?6 integrins is required for development and proper function of the UB. We further showed that when collecting duct (CD) cells lacking the ?3 or the ?6 subunit were plated on the same LM, there were major differences in integrin ?3?1- and ?6?1-dependent intracellular signaling. This result indicates that the ?3 and ?6 TM/CT domains activate specific signaling pathways. These observations pose several key unanswered questions in the field of LM integrins in the kidney: how do LM integrins synergize their signaling and what are the mechanisms whereby their TM/CT domains regulate differential signaling despite binding the same ligand? We will answer these questions by testing the hypothesis that functional and structural features of the TM/CT domains of the LM integrins confer specificity of signaling required for normal UB development and function. To test this hypothesis, we will: Aim 1) Define how the LM integrins cooperate in UB development and function. Aim 2) Determine the mechanism whereby integrin a3 and a6 CTs regulate CD cell function. Aim 3) Determine the structures of the individual a3, a6 TM/CT domains and of the ?3?1 and ?6?1 TM/CT heterodimers.