First Fluorescent Protein Identified in Vertebrates

Sophia David

A novel fluorescent protein discovered in Japanese eels may offer superior experimental advantages and clinical applications

In the early 1960s, researchers investigating the bioluminescent properties of the Aequorea victoria jellyfish discovered a protein that has since revolutionized experimental biology. The protein is, of course, green fluorescent protein (GFP).

In A. victoria, GFP works with a blue light-emitting bioluminescent protein called aequorin and together they convert Ca2+ induced luminescent signals into the characteristic green luminescence. Fluorescent proteins have also been discovered in other species, mainly microbes, jellyfish and corals. They have been engineered to produce light in a range of colours and brightnesses and are used extensively in cell biology to tag proteins or track gene expression. Together, they provide an invaluable toolbox for cell biologists.

A novel GFP

Last week, a group of Japanese researchers reported in the journal, Cell, that they have discovered the first known fluorescent protein in vertebrates. The protein, which they have named UnaG, was isolated from muscle fibres of the Japanese freshwater eel (Anguilla japonica), a common ingredient in Japanese cuisine known as Unagi.

UnaG is activated by a fundamentally different mechanism compared to all other known fluorescent proteins that have either constitutive fluorescence or covalent-bound ligands. Unusually, UnaG produces green fluorescence when activated by an endogenous ligand. This raises the possibility of engineering genetically encoded inducible fluorescent protein switches for a variety of applications.

Clinical applications

Remarkably, the endogenous ligand was identified as bilirubin, the major heme metabolite in animals and a common biomarker for liver function. Bilirubin is normally present at low concentrations in human blood but can accumulate if haemoglobin breakdown is increased or glucuronic acid conjugation is impaired. High levels of bilirubin in the circulation system and extravascular tissue can cause the prevalent childhood diseases, jaundice or kernicterus. Armed with knowledge about the structure of UnaG and its interaction with bilirubin, the researchers developed a fluorescence-based method for quantifying bilirubin from human clinical samples. The researchers believe that this approach could leader to simpler and more sensitive diagnostic tests requiring smaller blood samples.

UnaG is also unusual because, unlike Aequorea GFPs and other GFP-like proteins, it fluoresces brightly even when oxygen levels are low. It is thought that this could prove useful for visualising anaerobic environments inside cancer tumours.

UnaG could therefore provide significant experimental advantages compared to fluorescent proteins from lower organisms and, unlike any previously known fluorescent protein, offer medical applications. However, the discovery of UnaG in muscle fibres of eels also raises questions about its natural role and could help develop our understanding of muscle physiology.

A role in migration

During their life cycle, Japanese freshwater eels migrate thousands of kilometres from freshwater habitats into the Philippine Sea where they spawn. The larvae make a return journey over several months and mature back in the freshwater.

The researchers propose that UnaG may play a key role in facilitating these long distances migrations. They suggest that the interaction between bilirubin and UnaG may regulate the transport or availability of bilirubin, a known antioxidant and cytoprotectant in vivo. Bilirubin may reduce cellular oxidative stress, thus aiding the maintenance of anaerobic oxidative metabolism and steady-state muscle homeostasis and allowing continuous swimming. The team have since found that European and American freshwater eels (Anguilla Anguilla and Anguilla rostrata), which also swim long distances, also make UnaG.

Moreover, UnaG belongs to a family of proteins called fatty acid binding proteins (FABPs), members of which are found in humans and other animals. In particular, they are present in flight muscles of migratory birds and locusts where they are also thought to aid with long airborne journeys. It is thought that they prevent cell damage by reducing the excess accumulation of fatty acids, and act as a fatty acid transporter to aid efficient fuel usage.