YFPs

Yellow Fluorescent Proteins (YFPs) are a subclass of fluorescent proteins that emit light in the yellow portion of the visible spectrum. They are derived from the green fluorescent protein (GFP) originally isolated from the jellyfish Aequorea victoria. By introducing specific amino‑acid substitutions into the chromophore region, early YFP variants shifted the excitation and emission maxima toward longer wavelengths, typically around 514 nm excitation and 527 nm emission. This spectral shift enables simultaneous imaging of multiple targets with distinct fluorophores without significant spectral overlap.

YFPs are used extensively in cell biology, developmental biology, and biophysics for tagging proteins, monitoring gene expression, and measuring intracellular calcium and pH. Their brightness, photostability, and relatively low tendency to oligomerise compared to earlier variants make them suitable for live‑cell imaging. In addition to the wild‑type YFP, engineered variants have been developed to improve performance: mutants such as mYFP increase quantum yield, while ultra‑bright variants like YPet extend the usable light range for Förster resonance energy transfer (FRET) experiments. Some YFP variants have reduced maturation times or exhibit altered pH sensitivity, broadening their utility for diverse experimental conditions.

The use of YFPs has also expanded into optogenetics, where yellow‑shifted fluorescence allows for multiplexing with red‑ or blue‑shifted optical tools. Their compatibility with standard fluorescence microscopes and commercially available filter sets has made YFP a staple in molecular imaging kits. Researchers continue to refine YFP libraries, generating versions with altered subcellular targeting sequences or fusion tags to investigate protein–protein interactions, subcellular localization, and dynamic cellular processes.