Structure of a 14-3-3ϵ:FOXO3apS253Phosphopeptide Complex Reveals 14-3-3 Isoform-Specific Binding of Forkhead Box Class O Transcription Factor (FOXO) Phosphoproteins

Abstract

The transcriptional activity of Forkhead Box O3 (FOXO3a) is inactivated by AKT-mediated phosphorylation on Serine 253 (S253), which enables FOXO3a binding to 14-3-3. Phosphorylated FOXO3a binding to 14-3-3 facilitates the nuclear exclusion of FOXO3a, causing cancer cell proliferation. The FOXO3a/14-3-3 interaction has, therefore, emerged as an important therapeutic target. Here, we report a comprehensive analysis using fluorescence polarization, isothermal titration calorimetry, small-angle X-ray scattering, X-ray crystallography, and molecular dynamics simulations to gain molecular-level insights into the interaction of FOXO3apS253phosphopeptide with 14-3-3ϵ. A high-resolution structure of the fluorophore-labeled FOXO3apS253:14-3-3ϵ complex revealed a distinct mode of interaction compared to other 14-3-3 phosphopeptide complexes. FOXO3apS253phosphopeptide showed significant structural difference in the positions of the-3 and-4 Arg residues relative to pSer, compared to that of a similar phosphopeptide, FOXO1pS256bound to 14-3-3σ. Moreover, molecular dynamics studies show that the significant structural changes and molecular interactions noticed in the crystal structure of FOXO3apS253:14-3-3ϵ are preserved over the course of the simulation. Thus, this study reveals structural differences between the binding to 14-3-3 isoforms of FOXO1pS256versus FOXO3apS253, providing a framework for the rational design of isoform-specific FOXO/14-3-3 protein-protein interaction inhibitors for therapy.