p300 is a transcriptional adapter protein which is characterized by three cysteine-and histidine-rich regions and its C-terminus specifically binds the adenovirus E1A protein. p300, and p300-associated proteins, are components of TATA-binding protein (TBP) complexes. Protein kinase A mediated CREB phosphorylation results in the binding of CREB to a 265kDa nuclear protein designated CBP (for CREB-binding protein).CBP and p300 are homologous to each other.