Structural and Functional Characterization of FOXO3a in Transcription and Apoptosis

Forkhead box Class O (FOXO), one subfamily of the Forkhead box (Fox) family, which is featured by the Forkhead (FH) DNA-binding domain, includes four human transcription factors: FOXO1, FOXO3a, FOXO4, and FOXO6. The tumor suppressor FOXO3a is involved in multiple physiological and pathological processes, such as breast cancer and acute myeloid leukemia, and is related to human longevity. It plays essential role in metabolism, cell cycle arrest, DNA repair, and apoptosis. Besides the FH domain, FOXO3a contains three other regions (CR1-3), conserved within FOXO subfamily. It specifically binds a consensus Forkhead response element (FRE) DNA sequence through the FH domain, and recruits transcriptional coactivator CBP/p300 to activate gene transcription. FOXO3a functions through interacting with other proteins as well. FOXO3a binds p53 through the FH domain and the CR3 region, which are also engaged in an intramolecular interaction, and the solution structure of the former one was determined. This intramolecular interaction regulates coactivator recruitment and is disrupted by FRE DNA. A novel transactivation domain (TAD) CR2C was identified in addition to the known TAD CR3, both of which promiscuously associate with the KIX domain of CBP/p300 in equilibrium between two conformational states, the structures of which were determined by NMR spectroscopy. These two TADs of FOXO3a form additional multivalent binding to the TAZ1 and TAZ2 domains of CBP/p300, further increasing the promiscuity and complexity of the interaction. The coactivator recruitment is modulated by AMPK phosphorylation, which enhances the multivalent interaction between FOXO3a and CBP/p300, and thus the transactivation. These results indicate the significance of intrinsically disordered regions (IDRs) of FOXO3a in transcriptional activation and protein interaction, provide insight of the role of FOXO3a in gene transcription and apoptosis under various conditions, and potentially contribute to the cancer therapy.Ph