Summary I propose to develop and apply innovative hybrid structural biology tools to investigate the molecular mechanism of signaling through the T cell receptor (TCR) complex. This is a fundamental adaptive immunity pathway through which cytotoxic CD8+ T cells can detect the presence of viruses and developing tumors in the body. Immune function is achieved through the continuous surveillance of antigen-presenting cells for short peptides displayed within the molecules of the Major Histocompatibility Complex (MHC) on the cell surface. Key to the initiation of signaling, is a multi-subunit membrane protein assembly consisting of a clonotypic TCR heterodimer that recognizes the peptide-MHC, together with the invariant CD3 co-receptor hexamer that relays an activation signal from the cell surface intracellularly, through the plasma membrane. Besides the fundamental basic science merit, characterizing this process at atomic detail has important clinical implications, as is suggested by the large number of immunodeficiencies resulting from dysregulation of the signaling components and their interactions. Despite a large number of functional and structural studies, elucidating the 3D structure of the signaling complex as a whole remains extremely challenging by conventional methods, due to its size and dynamic complexity. As a result, the interactions between the TCR and CD3 subunits and the crucial conformational changes needed for signaling remain incompletely characterized. With these bottlenecks in mind, I have developed a new methodology combining datasets from complementary approaches, such as NMR, SANS and cryoEM, together with computational modeling using the program Rosetta to solve the structures of such challenging complexes. Here, I propose to apply this powerful integrative approach to elucidate the T cell receptor complex structure, and to study its dynamic transitions between inactive and active conformations. My immediate goal is to determine the molecular basis of TCR/CD3 interactions and to characterize the crucial structural changes in receptor complex arrangement upon antigen recognition. As a long-term goal, I plan to use a structure-guided approach to engineer novel T cell receptors with custom specificities and signaling properties, to be used in emerging immunotherapy applications.