FRET nanoscopy enables seamless imaging of molecular assemblies with sub-nanometer resolution

By circumventing the optical diffraction limit, super-resolved fluorescence microscopies enable the study of larger cellular structures and molecular assemblies. However, fluorescence nanoscopy currently lacks the spatiotemporal resolution to resolve distances on the size of individual molecules and reveal the conformational fine structure and dynamics of molecular complexes. Here we establish FRET nanoscopy by combining colocalization STED microscopy with multiparameter FRET spectroscopy. We simultaneously localize donor and acceptor dyes of single FRET pairs with nanometer resolution and quantitatively measure intramolecular distances with sub-nanometer precision over a large dynamic range. While FRET provides isotropic 3D distance information, colocalization measures the projected distance onto the image plane. The combined information allows us to directly determine its 3D orientation using Pythagoras's theorem. Studying two DNA model systems and the human guanylate binding protein hGBP1, we demonstrate that FRET nanoscopy unravels the interplay between their spatial organization and local molecular conformation in a complex environment.