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Teal fluorescent proteins: characterization of a reversibly photoswitchable variant

Fluorescent proteins (FPs) emerged in the mid 1990s as a powerful tool for life science research. Cyan FPs (CFPs), widely used in multicolor imaging or as a fluorescence resonance energy transfer (FRET) donor to yellow FPs (YFPs), were considerably less optimal than other FPs because of some relatively poor photophysical properties. We recently initiated an effort to create improved or alternate versions of CFPs. To address the limitations of CFPs, an alternative known as monomeric teal FP1 (mTFP1) was engineered from the naturally tetrameric Clavularia CFP, by screening either rationally designed or random libraries of variants. mTFP1 has proven to be a particularly useful new member of the FP 'toolbox' by facilitating multicolor live cell imaging. During the directed evolution process of mTFP1, it was noticed that some earlier variants underwent fast and reversible photoisomerization. Some of the initial characterization of one particular mutant, designated as mTFP0.7, is described in this manuscript.