Genetic Tag For Electron Microscopy

A new genetically encoded label for proteins could do for electron microscopy what green fluorescent protein has done for fluorescence microscopy, according to a report in PLoS Biology (DOI: 10.1371/journal.pbio.1001041). A research team led by chemistry Nobel Laureate Roger Y. Tsien of the University of California, San Diego, and Xiaokun Shu, now a pharmaceutical chemist at UC San Francisco, has developed miniSOG (mini singlet oxygen generator), a small fluorescent flavoprotein that can be expressed in a fused form with proteins of interest. Upon exposure to blue light and oxygen, miniSOG generates singlet oxygen, which oxidizes diaminobenzidine, also added to the tissue, causing the compound to polymerize locally. Staining with osmium—something the polymer soaks up readily—creates a small complex that can be detected by electron microscopy. The researchers demonstrated miniSOG's utility by identifying the sites in neurons where two previously unlocated synapse proteins reside. Although "the method is limited in that so far one can tag only one species of protein" at a time, says David DeRosier, a biologist at Brandeis University, "the tag is useful and will be a valued tool for cell biologists."