Structural insights into interactions of C/EBP transcriptional activators with the Taz2 domain of p300

Members of the C/EBP family of transcription factors bind to the Taz2 domain of p300/CBP and mediate its phosphorylation through the recruitment of specific kinases. Short sequence motifs termed homology boxes A and B, which comprise their minimal transactivation domains (TADs), are conserved between C/EBP activators and are necessary for specific p300/CBP binding. A possible mode of interaction between C/EBP TADs and the p300 Taz2 domain was implied by the crystal structure of a chimeric protein composed of residues 1723-1818 of p300 Taz2 and residues 37-61 of C/EBP epsilon. The segment corresponding to the C/EBP epsilon TAD forms two orthogonally disposed helices connected by a short linker and interacts with the core structure of Taz2 from a symmetry-related molecule. It is proposed that other members of the C/EBP family interact with the Taz2 domain in the same manner. The position of the C/EBP epsilon peptide on the Taz2 protein interaction surface suggests that the N-termini of C/EBP proteins are unbound in the C/EBP-p300 Taz2 complex. This observation is in agreement with the known location of the docking site of protein kinase HIPK2 in the C/EBP beta N-terminus, which associates with the C/EBP beta-p300 complex.