The pathway of gene regulation linking hormone receptor activation to the induction of cAMP-regulated endocrine genes is one of the best characterized transcriptional pathways. A key process in this pathway is the phosphorylation of the transcription factor CREB by the catalytic subunit of the cAMP-dependent protein kinase A (PKA). The principal investigator has proposed that this phosphorylation event allows CREB to recruit a co-activator protein, CBP, which is responsible for transcriptional activation. This model was recently extended by the finding from another laboratory that phosphorylated c-jun also functions through CBP. These findings, along with the newly appreciated relationship between CBP and the adenovirus E1A-associated protein p3OO, may explain how E1A inhibits the expression of cAMP- and phorbol ester- regulated genes and leads to cellular immortalization. The specific aims of this proposal are to: l) Rigorously determine whether c-jun phosphorylated by jun kinase interacts with the CREB-binding domains of CBP and p3OO. Fluorescence anisotropy measurements will be performed to compare the binding of phosphorylated c-jun and CREB to CBP and p3OO under equilibrium, solution- phase conditions to test whether competition for CBP/p300 binding explains the phenomenon of CREB/jun "squelching." 2) Test whether HTLV-1 Tax stimulates cAMP-mediated gene expression by augmenting the association of phosphoCREB and CREM with CBP. 3) Determine whether tissue- and developmental stage-specific Ets-domain proteins that interact with the "activation" domains of CBP and p3OO contribute to the process of synergistic signaling. 4) Determine the role of the CBP/p300 bromodomain and the proteins that bind to this region.