YPet

YPet is a monomeric yellow fluorescent protein derived from the jellyfish *Aequorea victoria* fluorescent protein family. It was engineered by directed evolution to exhibit improved photostability, brightness, and a higher quantum yield compared with the original GFP, making it useful for live-cell imaging and protein–protein interaction studies. YPet emits in the yellow spectral range, with an excitation maximum near 516 nm and an emission maximum near 527 nm, which allows it to be excited simultaneously with green fluorescent proteins while emitting a distinct signal.

The protein was first reported in 2009 by C. Pascal and colleagues, who performed error-prone PCR and site‑directed mutagenesis on the Aequorea GFP scaffold to generate YPet. Subsequent characterization revealed a brightness about 130 % greater than that of the green fluorescent protein variant EGFP and a significantly reduced tendency to oligomerize. YPet also displays improved resistance to photobleaching and a higher tolerance to the addition of fusion partners, permitting its use as a fluorescent tag in a broad range of organisms from yeast to mammalian cells.

YPet has been incorporated into several widely used biological tools. It is frequently employed as a FRET acceptor in pairs such as CFP–YPet or Aequorea CFP variants. Its spectral properties also make it compatible with fluorescent lifetime imaging microscopy (FLIM) and resonance energy transfer assays. Additionally, YPet has been fused to proteins of interest to monitor trafficking, localization, and complex formation in vivo, as well as in the construction of biosensors that report on intracellular pH, calcium, or enzyme activity.

Because of its favorable physical and optical characteristics, YPet remains a valuable resource in cell biology, biochemical assays, and structural biology, enabling researchers to visualize and quantify dynamic processes with high temporal and spatial resolution.