Approximately one-third of the proteins in the human proteome are intrinsically disordered (IDPs). IDPs play a central role in cellular signaling and regulatory pathways and are directly implicated in numerous devastating diseases such as cancer, leukemia, diabetes, and neurodegenerative disease. Because of the central role of IDPs in signaling and regulation and the unique features of their complexes with their physiological partners, interactions involving disordered proteins are frequently targeted by viral proteins that mimic cellular IDP motifs. Many regulatory IDPs function through the transient assembly of multi- component signaling complexes, where flexibility and dynamic exchange of binding partners plays a critical role. The dynamic nature of IDPs and their regulatory complexes imposes a major challenge to traditional structural biology techniques. The overarching goal of this project is to characterize the conformational ensemble and interactions of a key intrinsically disordered transcription factor, the cyclic AMP response element binding protein (CREB), to elucidate the molecular mechanism by which its activity is down regulated by hyperphosphorylation in response to DNA damage and by binding to an important human T cell leukemia virus oncoprotein that promotes lymphoid cell transformation and progression of leukemia. CREB plays a critical role in regulating normal hematopoietic stem cell differentiation and is implicated in proliferation of myeloid and acute leukemias. Transcription of CREB- responsive genes is regulated by phosphorylation at multiple sites and by interactions with the CBP/p300 and CRTC coactivators. A powerful combination of complementary biophysical tools - NMR, single molecule FRET (smFRET), and small angle X-ray scattering (SAXS) – will be used to characterize the conformation of CREB and conformational changes associated with hyperphosphorylation and with binding to DNA, coactivators, and the basic leucine zipper oncoprotein (HBZ) of human T-cell leukemia virus type 1.