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Professor Alice Ting and colleagues have developed a tool that involves genetically fusing an engineered ascorbate peroxidase (APEX2) to a protein of interest to catalyze the production of an electron-dense precipitate that is easily visualized with either light or electron microscopy. This reaction occurs quickly and locally, thereby attaining very high spatial resolution. The tag is non-toxic and functional in all cellular compartments. Furthermore, it can be fused to fluorescent proteins, allowing for images from electron microscopy to be correlated with those from fluorescence microscopy.

APEX2 has been demonstrated for EM imaging of a wide variety of cellular proteins in virtually every subcellular compartment. APEX2 is also useful as a marker of subcellular organelles. Although APEX2 has been most extensively used in cultured cells, it has also been successfully employed in a variety of model organisms, including C. elegans, Drosophila, mice, and zebrafish. See the references for details.