Project Summary Physical integrity of epithelial tissue is established and maintained by the cytoskeletal network which integrates cells into their environment with other neighboring cells and the extracellular matrix. In particular, keratin intermediate filament proteins ubiquitously expressed in epithelial cells are responsible for the structural integrity of epithelial tissues and recently emerged as a driver of collective cell migration. Yet, unlike actin, understanding of keratin force-sensing is still very limited. We discovered that the keratin network responds to externally applied physical forces by recruiting cten, a protein known to act as both a tumor suppressor and promoter in a tissue-specific manner. Emerging evidence indicates that more proteins are recruited to the force-bearing keratin fibers, suggesting that the keratin network may serve as a critical hub for mechano-transduction. The overall premise of this application is that discovering the basic mechanisms and functions of keratin-based mechano-sensing will contribute to the understanding of how the cell senses forces. Our goal is to determine the force-dependent protein interactome surrounding the keratin network in epithelial cells and how keratin-mediated mechano-transduction impacts cell behavior. To resolve force-sensitive protein-protein interactions, we will inscribe proximal proteins by in situ promiscuous biotin labeling while cells are being physically stimulated. Newly identified candidates will be tested to verify their force-dependent co- localization with keratin filaments in live cells and in vitro, and analyzed for their roles in transcriptional regulation, cell integrity maintenance, and collective cell migration. Our approach will reveal the comprehensive list of keratin-associated proteins in the presence or absence of external forces and, for the first time, resolve the force-dependent regulation of the keratin network and its physiological implications.