Ser715 Phosphorylation Induces β-Hairpin Unfolding and Destabilization of FOXM1 Autoinhibition

FOXM1 is a cell proliferation-driving transcription factor activated by phosphorylation-induced conformational changes. In its inactive state, an intramolecular β-hairpin within the transactivation domain (TAD) binds the N-terminal repressor domain (NRD), forming a composite β-sheet that locks the protein in an autoinhibited conformation. Despite the known importance of this regulatory switch, the molecular events that unlock FOXM1 remain poorly characterized. Here, we performed 5 μs all-atom molecular dynamics simulations of human FOXM1b NRD-TAD complexes in both unphosphorylated and tetra-phosphorylated states, modeling four experimentally validated regulatory phosphosites. Our results showed that phosphorylation induces local unfolding of the β-hairpin beginning at Ser715, located at the hairpin turn, and propagates to global disruption of the NRD interface through hydrogen bond loss, salt bridge rupture, and secondary structure collapse. In contrast, the unphosphorylated complex maintains stable hairpin geometry and interdomain contacts. Additional replicate tetra-phosphorylated simulations and a monophosphorylated Ser715 simulation reproduced the β-hairpin unfolding event, supporting both reproducibility and the sufficiency of Ser715 phosphorylation in initiating this transition. Per-residue MM-PBSA energy decomposition further reveals that phosphorylation redistributes interdomain interaction energetics, with Ser715 emerging as the dominant locus of energetic perturbation despite the presence of multiple phosphosites. Together, these findings support a phosphorylation-triggered order-to-disorder transition that relieves FOXM1 autoinhibition and highlight Ser715 as a key structural and energetic switch. Our study provides a dynamic molecular framework for targeting FOXM1 activation via its regulatory fold.