Project Summary/Abstract Our current understanding of in vitro protein folding is due to decades of experimental and computational research that provided high-resolution characterization of protein structure, identification of folding principles, and development of folding algorithms. However, proteins often participate in new and unexpected functional and pathological behaviors in vivo. Because protein processes involve a large network of interactions that strongly depend on the environment, understanding how proteins work inside cells requires knowledge of protein structure, stability, and dynamics in vivo. While evidence that the cellular environment perturbs protein behaviors emerged over half a century ago, we still have limited fundamental information about the effects of these cooperative cellular interactions on protein properties. The gap in knowledge is largely attributable to the weak transient nature of interactions in the cellular milieu and challenges associated with studying protein functions in living cells. This limitation is concerning because proteins in cells and organisms are continuously interacting with other biomolecules, which may disrupt the ability of a protein to fold and assemble properly and results in loss of function and eventually disease. To address these gaps, our research group leverages groundbreaking in vivo spectro-microscopy methods, in combination with functional biochemical assays, in vitro biophysical spectroscopy, and numerical analysis solutions to characterize protein structural dynamics in living cells and tissues. This platform will transform our ability to examine unexplored layers of protein complexity and regulation in cells and tissues, specifically: (1) Do classic in vitro protein principles translate to cells? How accurate are the in-cell predictions of folding theory and molecular dynamics simulations? (2) Can we develop methods to visualize the spatial distribution of metabolism and associated metabolic protein structural dynamics in living cells? (3) How does thermal adaptation and acclimation by organisms change the stability, folding, and aggregation of proteins in differentiated tissues? Overall, this work will lead to a greater understanding of how remodeling of the cell interior during development, environmental stress, and disease contributes to protein homeostasis. Unraveling these interactions will improve our molecular-level understanding of essential processes in human health and disease.