Fluorescent proteins have been isolated from invertebrate species only, until now. A group of researchers from Japan isolated a green fluorescent protein from the freshwater eel called Unagi (yes, the same Unagi used for sushi).
The protein, named UnaG, is smaller than GFP (139 amino acids compared to 237 of GFP), excited at 498nm (after bilirubin binding) and emits light at 527nm.
UnaG is a green fluorescent protein found in eel muscle. Source: Kumagai et al. (2013) Cell 153(7):1602-1611
UnaG, glows in green upon noncovalent binding to bilirubin – a membrane permeable heme metabolite. This is a major advantage, since this mechanism can be utilized as a fluorescent switch: add bilirubin–> get fluorescence; remove bilirubin–>remove fluorescent.
This unique characteristic of UnaG prompted the researchers to develop a sensitive assay to measure bilirubin levels in blood serum – a known biomarker for several human diseases. Their assay sensitivity is 100-fold better than current clinical assays, they claim.
Another big advantage of this protein is that its fluorescence is independent of oxygen (unlike GFP-based FPs). UnaG can therefore be used under anaerobic conditions.
UnaG fluorescence depends on bilirubin, but not on oxygen. Source: Kumagai et al. (2013) Cell 153(7):1602-1611
The biological role of UnaG is still unknown, but it is suggested to have a function in oxidative stress.
I think that this is just the opening shot for the search for more vertebrate fluorescent proteins…
Read a research highlight from Nature Methods.
Unagi, from “Friends”:
Kumagai A, Ando R, Miyatake H, Greimel P, Kobayashi T, Hirabayashi Y, Shimogori T, & Miyawaki A (2013). A bilirubin-inducible fluorescent protein from eel muscle. Cell, 153 (7), 1602-11 PMID: 23768684
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