The Story of Green Fluorescent Protein

This is a special guest blog article by Joel Lusk, a third-year PhD student in Biological Design at Arizona State University and a member of the Science on Main group. His research involves the study of photoacoustic contrast agents in the imaging and detection of cells. He can be reached at jlusk@asu.edu.

Have you ever seen the stories in the news where scientists make plants or animals glow? These stories are fascinating and amazing, and scientists accomplish these feats by using a protein that was originally found in a jellyfish! This protein is called Green Fluorescent Protein (GFP for short), and the story of its discovery and the scientists that originally pioneered its use is an interesting one. The protein itself has been used in science in a wide variety of applications to study plants, biological processes, and even the things that make up our cells.

Two mice expressing enhanced GFP with one non-GFP mouse in the
center. Source: Moen et al., 2012, BMC Cancer 12.1: 21.

 In 1960, a scientist named Osamu Shimomura started studying the bioluminescence of the jellyfish species Aequorea victoria. When the rings of these jellyfish are squeezed, they produce a glowing liquid. Shimomura claimed to have squeezed over a million jellyfish to collect this liquid and study it for his research. In 1962, he and his colleagues was the first to identify a protein in this jellyfish that he named aequorin. The aequorin first emits blue light, which is then absorbed by the GFP, causing it to glow.

The Aequorea victoria jellyfish. Copyright: Sierra Blakely.

Now that the protein that was responsible for the jellyfish's glow was identified, several other scientists became interested in cloning the gene responsible for it. If the gene for the fluorescent protein could be cloned and inserted into other organisms, it could revolutionize biology. In 1992 a biologist named Douglas Prasher suggested that this glowing protein could trace the way proteins moved in cells and was the first to clone the GFP gene. Unfortunately, the funding for his lab ran out and he left academia. Before he left, Prasher sent several samples of GFP to other scientists so they could continue his work.

After the genetic sequence was found for this glowing protein and it had been successfully cloned, the doors opened to a world of new possibilities for GFP. Prasher had predicted that this protein could be used as a reporter molecule in cells. This means that when a scientist wanted to know if a cell was producing a protein, they could just add the gene for GFP next to the gene for the other protein into the genetic code. When the cell produced that protein, it would also produce GFP, causing the cell to glow. This allowed scientists to undertake all sorts of amazing experiments, including the work of Martin Chalfie, who produced these proteins in E. coli bacteria and worms called C. elegans.

Neurons imaged from a "Brainbow" mouse. Source: Smith
(2007), Opinion article in Neurobiology 17.5: 601–608.

Another scientist, Roger Tsien, took the GFP and augmented it to be many different colors. He did this by changing the shape of the protein so it absorbed and emitted light at different wavelengths. This allowed scientists to not just label cells and animals with a green protein, but with an entire rainbow of colors! This has been applied to every part of the field of biology, allowing scientists to understand and see processes they never could before. Shimomura, Chalfie, and Tsien all shared the 2008 Nobel Prize in Chemistry for their work with GFP. Unfortunately, Prasher, the man who first cloned the protein, was not awarded the prize as three people maximum can share the prize. To honor him, all three recipients mentioned him in their acceptance speeches as even invited Prasher to the ceremony.

So now we know the history of GFP, but what are some of the cool things that scientists can do with it? Mainly scientists have used the protein to tag and visualize processes in cells. One amazing application of GFP and other fluorescent proteins was the production of a "Brainbow" mouse. This mouse produces a variety of colors in its neurons, allowing scientists to see the way the neurons interact with each other. Not only does this tell scientists a lot about how the brain works, it also creates a beautiful image!

The story of GFP is one that shows the usefulness of a fundamental science. Who would have thought that a protein first found in a jellyfish would go on to revolutionize the way we understand and see cells? Research into fluorescent proteins is still going strong, and scientists are currently looking at new fluorescent proteins that have been isolated from other sources such as bacteria and coral. Further work with the fluorescent proteins might reveal even more about the inner workings of our cells and our bodies. In the meantime, we can have a little fun with these proteins, and even paint pictures using bacteria who produce these fluorescent proteins across the visible light spectrum!

A picture painted on an agar plate with bacterial colonies expressing a
variety of fluorescent proteins. Source: Tsien Lab. Artwork: Nathan Shaner.
Photographer: Paul Steinbach.

Sources:

https://www.conncoll.edu/ccacad/zimmer/GFP-ww/shimomura.html

Chalfie, Martin, et al. "Green fluorescent protein as a marker for gene expression." Science 263.5148 (1994): 802-805.

Moen, Ingrid, et al. "Gene expression in tumor cells and stroma in dsRed 4T1 tumors in eGFP-expressing mice with and without enhanced oxygenation." BMC cancer 12.1 (2012): 21

Tsien, Roger Y. "Constructing and exploiting the fluorescent protein paintbox (Nobel Lecture)." Angewandte Chemie International Edition 48.31 (2009): 5612-5626.

Smith, Stephen J. "Circuit reconstruction tools today." Current opinion in neurobiology 17.5 (2007): 601-608.

http://www.tsienlab.ucsd.edu/Images.htm