Infrared Fluorescent Protein Extends Imaging Range

The best wavelengths for optical imaging in animal tissues are between 650 and 900 nm because those wavelengths avoid background interferences and allow penetration of light through the tissue. But fluorescent proteins, which are commonly used to label proteins of interest, have not been available in this wavelength range, until now. Roger Y. Tsien of the University of California, San Diego, and coworkers report an infrared fluorescent protein engineered from a bacterial phytochrome that can be expressed in mammalian cells (Science 2009, 324, 804). When expressed, the protein incorporates biliverdin IXα as a chromophore and exhibits excitation and emission peaks at 684 nm and 708 nm, respectively. Biliverdin, which is an intermediate in the natural breakdown of heme, is readily available in mammals but has negligible fluorescence on its own. The engineered protein fluoresces brightly when expressed in human embryonic kidney cells, and adding exogenous biliverdin boosts the fluorescence when endogenous biliverdin has not already saturated the protein. The researchers used the protein to image human embryonic kidney cells and the livers of living mice.