Structures of KIX domain of CBP in complex with two FOXO3a transactivation domains reveal promiscuity and plasticity in coactivator recruitment

摘要

Forkhead box class O 3a (FOXO3a) is a transcription factor and tumor suppressor linked to longevity that determines cell fate through activating transcription of cell differentiation, survival, and apoptotic genes. Recruitment of the coactivator CBP/p300 is a crucial step in transcription, and we revealed that in addition to conserved region 3 (CR3) of FOXO3a, the C-terminal segment of CR2 (CR2C) binds CBP/p300 and contributes to transcriptional activity. CR2C and CR3 of FOXO3a interact with the KIX domain of CBP/p300 at both "MLL" and "c-Myb" binding sites simultaneously. A FOXO3a CR2C-CR3 peptide in complex with KIX exists in equilibrium between two equally populated conformational states, one of which has CR2C bound to the MLL site and CR3 bound to the c-Myb site, whereas in the other, CR2C and CR3 bind the c-Myb and MLL sites, respectively. This promiscuous interaction between FOXO3a and CBP/p300 is further supported by additional binding sites on CBP/ p300, namely, the TAZ1 and TAZ2 domains. In functional studies, our structure-guided mutagenesis showed that both CR2C and CR3 are involved in the activation of certain endogenous F0X03a target genes. Further, phosphorylation of S626, a known AMP-dependent protein kinase target in CR3, increased affinity for CBP/p300 and the phosphomimetic mutation enhanced transactivation of lucifer-ase. These findings underscore the significance of promiscuous mul-tivalent interactions and posttranslational modification in the recruitment of transcriptional coactivators, which may allow transcription factors to adapt to various gene-specific genomic and chromatin structures and respond to cell signals.