High-Speed Atomic Force Microscopy Reveals Single-Molecule Dynamics of Fulvestrant-Bound Estrogen Receptor α

Fulvestrant is a selective estrogen receptor downregulator (SERD) that antagonizes estrogen receptor α (ERα) and also promotes receptor degradation. However, how it affects full-length ERα dynamics at the single-molecule level remains unclear. Here, we used high-speed atomic force microscopy (HS-AFM) to directly visualize the dynamic behavior of full-length ERα under fulvestrant-bound, ligand-free, and other ligand-bound conditions. Among the four conditions, fulvestrant-bound ERα alone showed a reduced apparent molecular volume and increased surface mobility, behaving distinctly from the ligand-free, E2-bound, and 4-OHT-bound states. These results indicate that fulvestrant not only drives ERα inactivation and degradation but also is associated with changes in the observed molecular morphology and dynamics of full-length ERα. Our study provides direct single-molecule observation of altered molecular behavior under fulvestrant-bound conditions, which we interpret as reflecting changes in the conformational ensemble of full-length ERα.