The ability to image living cells and tissues has revolutionized our understanding of many biological processes. Being able to visualize what’s happening in these time lapse movies at an ultrastructural level would make them even more informative. But commonly used fluorescent proteins like GFP are not directly visible by electron microscopy (EM). Fluorescent nanoparticles or quantum dots can be visualized by EM, but targeting these to cytoplasmic proteins in living cells remains difficult. One method which does allow correlation between light microscopy and EM is fluorescence photoconversion, where observation of the fluorescent label causes the deposition of a reaction product that can be rendered electron-dense and directly visualized by EM. In the January issue of Cold Spring Harbor Protocols, Mark Ellisman and colleagues from UCSD’s National Center for Microscopy and Imaging Research provide a set of articles for this method in combination with a class of genetically encoded peptide tags that can be labeled in living cells by fluorophores bearing two appropriately spaced arsenic atoms (biarsenicals).

Correlated Live Cell Light and Electron Microscopy Using Tetracysteine Tags and Biarsenicals provides an overview of the technique, and protocols are provided for Labeling Tetracysteine-Tagged Proteins with Biarsenical Dyes for Live Cell Imaging and Fluorescence Photoconversion of Biarsenical-Labeled Cells for Correlated Electron Microscopy (EM).