b-catenin is a bi-functional protein that plays essential roles in Wnt-mediated transcription and cadherin- based intercellular adhesion. Since b-catenin binding to cadherin mediates tumor suppression, while b- catenin transcriptional function drives cellular transformation, understanding what controls b-catenin targeting to transcriptional or adhesive complexes will be relevant towards considering strategies that seek to inhibit the oncogenic, but spare the tumor suppressor activities of b-catenin. Our preliminary studies show that a form of b-catenin can be generated that preferentially binds to the transcription factor, T-Cell Factor (TCP), but not cadherin-type adhesion receptors. This signaling form is monomeric and is regulated by the C-terminus of b-catenin, which we propose selectively competes cadherin binding through an intramolecular fold-back mechanism. In contrast, the main cadherin-binding form of b-catenin is a b-catenin/a-catenin dimer, indicating that there is a distinct molecular form of b-catenin that can interact with both the cadherin and a-catenin. The overall objective of this proposal is to determine how cytoplasmic regulation of b-catenin dictates adhesive versus signaling functions, which is relevant to b- catenin's dual role as a tumor suppressor and oncogene. We hypothesize that the terminal regions of b- catenin direct these distinct functions of b-catenin. Towards this end, we propose to identify the sequences of b-catenin that control its binding to cadherin versus TCP using an in vitro, affinity-binding assay, together with a conformation-specific antibody and mutagenesis approaches (Aim 1). The role for phosphorylation in regulating b-catenin binding selectivity will be determined using phosphatase and phosphopeptide mapping techniques (Aim 2). Furthermore, how the C-terminus of b-catenin modulates adhesive function in vivo will be determined using a cell attachment assay that specifically quantifies cadherin adhesive activity (Aim 3). These experiments will help us understand how b-catenin targeting to cadherins and TCP transcriptional complexes is regulated, which will provide insights into how cells coordinate adhesive and signaling functions of b-catenin throughout normal development and tumor progression.
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