Green fluorescent protein (GFP) is protein that occurs in a species of jellyfish, Aequorea victoria, which is found in the North Pacific. Fluorescence is a phenomenon whereby certain substances absorb energy from electromagnetic radiation, such as light, and emit the energy at a different, normally longer, wavelength. The green glow produced by GFP results from it absorbing relatively high-energy blue and ultraviolet light and emitting it as green light, which has a longer wavelength and less energy; it will therefore glow green when exposed to invisible ultraviolet light. GFP is of particular interest to biologists as, unlike most other fluorescent proteins, it fluoresces by itself without the requirement for any interaction with other molecules. Since it is a protein made up entirely of amino acids, this means that organisms can be genetically engineered to produce it, giving rise to a wide range of applications in various fields of biology.
Bioluminescence occurs in many marine organisms. In the case of Aequorea victoria, a chemiluminescent substance called aequorin emits blue light when it combines with calcium ions. This light is then absorbed by the green fluorescent protein to produce a green glow. A number of other marine organisms have been found to contain these substances, but it is not clear why they have evolved to produce this glow or to change the color from blue to green. One suggestion, based on experimental evidence that glowing GFP can release electrons, is that GFP could act as a light-activated electron donor, in a similar way to chlorophyll in green plants.
The green fluorescent protein has a complex structure. The fluorescent part — known as a fluorescent chromophore — consists of three amino acids, tyrosine, glycine and either serine or threonine, joined in a ring shape. This is contained within a cylindrical structure that protects the chromophore from contact with other molecules, a feature that is crucial to the fluorescence, as contact with water molecules would otherwise dissipate the energy used to produce the green glow.
GFP has proved to be extremely useful in fields such as genetics, developmental biology, microbiology and neurology. It can be used for tagging specific proteins within an organism in order to see where and when they are expressed; the part of the organism’s DNA that codes for the protein of interest can be engineered to also synthesize GFP, thus allowing tracking of the protein within living cells using ultraviolet light. Viruses can also be tagged in this way, allowing infections in living organisms to be monitored. Green fluorescent protein can also be modified to fluoresce in several other colors, opening up new possibilities. One of these has been the creation of transgenic mice with varying combinations of fluorescent proteins expressed in neurons, which allow neural pathways in the brain to be studied in detail.
Other applications have been found outside biology. One controversial development is the engineering of fluorescent pets. Genetically engineered animals that produce green fluorescent protein have been created, and include fish, rats, pigs and a rabbit.