UnaG, a new green fluorescent protein from japanese eel

Unagi is a often seen dish in japanese cuisine, yet it never occurred to me that the eel can be fluorescent. Well, indeed it is. As Atsushi Miyawaki discovered this bilirubin binding fluorescent protein in eel muscle, for the very first time, a fluorescent protein in vertebrate is identified.

The green fluorescence of UnaG is from binding of a endogenous chromophore ligand called bilirubin(BR). This is very different from Jellyfish Green Fluorescent Protein(GFP), which is self-sufficient to form a chromophore by a series of autocatalytic post-translational modification. Thus GFP and its homologues are also referred as auto fluorescent proteins (AFP).

Apart from AFPs, yet exists another type of fluorescent proteins I would like to call chromophore-binding fluorescent protein, such as IFP and iRFP. They are both near-infared fluorescent proteins engineered from biliverdin (BV) binding bacterial phytochrome. Much like UnaG, these proteins show no fluorescence at apo state, upon proper ligand binding, intensive fluorescence will be induced.

Compare to GFP, which is oxygen-dependent in chromophore formation, the advantage of UnaG is that it can be used under both aerobic and anaerobic conditions. And the brightness of UnaG in ligand bound state is also better than EGFP on numbers. These make UnaG a tempting alternative of GFP in some research areas. Due to the high-affinity and high specificity bilirubin binding property of UnaG, it was developed as a bilirubin sensor in the study.

Looking into the future, it is almost certain that a lot more fluorescent proteins like UnaG will be identified in vertebrates. As for UnaG itself, one can imaging it will be served as a prototype for engineering bight, stable fluorescent proteins, even with alternative colours.

Reference:

1.Kumagai, A. et al. A Bilirubin-Inducible Fluorescent Protein from Eel Muscle. Cell 1–10 (2013). doi:10.1016/j.cell.2013.05.038