Light microscopy of proteins in their ultrastructural context

Resolving the distribution of specific proteins at the nanoscale in the ultrastructural context of the cell is a major challenge in fluorescence microscopy. We report the discovery of a new principle for an optical contrast equivalent to electron microscopy (EM) which reveals the ultrastructural context of the cells with a conventional confocal microscope. By decrowding the intracellular space through 13 to 21-fold physical expansion while simultaneously retaining the proteins, bulk (pan) labeling of the proteome resolves local protein densities and reveals the cellular nanoarchitecture by standard light microscopy. Imaging specific proteins in the ultrastructural context largely relies on correlative light/electron microscopy, but fluorophore incompatibility and registration issues limit its use. Here the authors develop an expansion microscopy method with pan-labeling of the proteome to obtain EM-equivalent light microscopy images.