In the twilight depths off the west coast of North America lives a small and graceful jellyfish floating apparently aimlessly through the void. Who would have known that this humble jelly—Aequorea victoria—was set to revolutionise cellular biology in the latter half of the twentieth century. Along the rim of the jellyfish’s bell (the propulsive body) lies a ring of light-emitting organs which, in the blackness, produce an electric green glow that wouldn’t be out place in a Ghostbusters film. This luminescence can be attributed to a chemical mechanism based around the molecule known as the Green Fluorescent Protein (GFP), synthesised by the jellyfish. Earning those involved in its discovery the Nobel Prize in 2008, GFP has been the key to unlocking the potential of biological imaging over the last 25 years.
The light organ houses two molecules essential for the light reaction: aequorin and GFP, working in conjunction. By catalyzing the degradation of the protein luciferin, aequorin causes blue light to be released. Rather than emitting this blue light, the photons are instead used an as energy source to activate the fluorescence of GFP. GFP has an excitation peak at the wavelengths of 395 nm and 475 nm—corresponding to blue and UV light. This means that it will most efficiency absorb light in this range of the spectrum. Absorbing this light leaves GFP in an unstable state with ‘too much’ energy, being described as excited. Emission of green light at the wavelength of 508 nm, energetically lower than that it absorbed, returns it to its stable state.
Green light is rare in the ocean depths, meaning that an organism that can luminesce in such a way will be more obvious in its surroundings, allowing it to attract prey and confuse predators. But how is this relevant to cell biology in the laboratory? In 1992, American scientist Douglas Prasher sequenced and cloned the wild-type GFP gene. Over the following few years GFP became the darling of molecular genetics, a result of our ability to fuse the gene onto the beginning or the end of any other gene in any organism.
If inserted into an embryo, every cell in the body can inherit the GFP tagged protein. When the resulting organism is exposed to UV light it then glows green. This allows scientists to track both the distribution and the concentration of the protein throughout individual cells or through the organism as a whole, depending on which protein is tagged with GFP. We can see the trafficking of the proteins through the cell in real time, highlighting a host of cellular processes from protein packaging to the structure of the nuclear membrane.
Over the course of its history GFP has been constantly engineered and modified, transforming it into an increasingly more effective and versatile tool. A whole spectrum of different colours of fluorescent proteins have now been engineered. By using a red-producing variant of GFP, scientists have found success in diagnosing cancer since, due to its longer wavelength, red light can travel further through intervening tissue.
On a grander scale, one couldn’t discuss GFP without bringing up the glow-in the dark rats, cats, rabbits, pigs, monkeys…you name it. Due to its obvious but relatively benign nature, GFP serves as one of the earliest genes used when trialling an organism with genetic modification, as a proof of the technology before more complex manipulation is attempted, with wide implications especially within medicine. We will soon reach the point where we can easily extract vaccines from cow’s milk, and produce disease resistant pigs.
The story of a simple jellyfish that has gone onto transform the very nature of molecular biology and medicine is a testament to the resourcefulness of science and humanity as a whole. It proves that the most useful of tools can have the most unlikely of origins, and should serve as a needed reality check. With every extinction, we say goodbye to another jewel in the biological crown, the vast wealth of unique genetic information that the organism possessed vanishing often forever. Who knows how many ‘GFP’s’ we’ve already lost.