fluoroprotein

A fluoroprotein is a protein that exhibits intrinsic fluorescence because it contains or forms a fluorophore as part of its structure. The most familiar examples are fluorescent proteins (FPs), such as the green fluorescent protein (GFP) and its numerous variants, originally discovered in jellyfish and corals. These proteins generate light not by binding a separate dye but by forming a chromophore from specific amino acids within the protein itself.

The fluorescence of many fluoroproteins arises through autocatalytic maturation: a short sequence of amino acids within the protein is converted into a chromophore that then absorbs and emits light. GFP-like fluoroproteins typically require molecular oxygen for chromophore maturation. The protein’s beta-barrel structure protects the chromophore and contributes to stability and brightness.

Fluoroproteins are widely used as genetically encoded reporters. By encoding the fluoroprotein gene as a fusion with a protein of interest or placing it under a specific promoter, researchers can visualize gene expression, protein localization, and dynamic cellular events in living cells and organisms. A large set of spectral variants exists, enabling multi-color labeling and quantitative measurements. Engineered fluoroproteins include monomeric forms to reduce aggregation, brighter or more photostable variants, and sensors that change fluorescence in response to ions or environmental conditions. Notable applications include calcium indicators, pH sensors, and voltage indicators, as well as photoconvertible or photoswitchable proteins for lineage tracing and super-resolution techniques.

Limitations include dependence on oxygen for maturation, potential photobleaching, spectral overlap in multi-color experiments, and the need for appropriate expression levels to avoid perturbing biology. Near-infrared fluoroproteins derived from phytochromes expand imaging depths in tissues.