Jellyfish and Glowing Proteins

By: Erica Gorenberg

From the stars on the ceiling of a childhood bedroom to the key chains brought home as souvenirs, some of the most memorable trinkets from youth are those that glow in the dark. For molecular biologists, like for kids, the ability to harness fluorescence and make molecules reminiscent of those glow-in-the-dark toys was one of the most useful and exciting innovations in modern science.

Green fluorescent protein (GFP) was first observed in 1962 in a bioluminescent jellyfish, Aequeorea victoria, as the molecule responsible for the animal’s ability to glow. The protein was isolated, and researchers demonstrated its ability to light up green under beams of specific colors of light. The use of GFP revolutionized biological research–in the time since GFP’s discovery and use, molecular biology has entered its golden age.

Osamu Shimomura, who first isolated the protein, Martin Chalfie, who first used GFP to track other proteins, and Roger Tsien, who discovered the properties that make GFP fluorescent and manipulated them to create a rainbow of fluorescence proteins, were awarded the 2008 Nobel Prize in Chemistry. Before their work with GFP, visualizing proteins within live cells was far more complex, and less dependable, as it relied on the insertion of fluorescent dyes into the cell where they could bind to the proteins of interest. These dyes were unreliable because they weren’t always specific to their proteins, and because the physiology of the cell had to be disrupted to add them.

Now, through the use of DNA modification technologies, the gene for GFP can be fused to genes of other proteins, allowing proteins to be produced with the fluorescent molecule attached. This is called "tagging" the protein with GFP. As Martin Chalfie demonstrated for the first time in 1994, GFP can be used to show specific proteins and cellular structures in living organisms, providing researchers with new insights into cellular function. For example, when the GFP gene is attached to the gene for a microtubule associated protein, MAP2, one molecule of GFP is produced and fused with every molecule of MAP2. By viewing cells that produce MAP2, such as neurons as shown in Figure 1, researchers can learn how much of the protein is expressed under different conditions, simply by measuring the intensity of GFP’s fluorescence. The more MAP2 that’s produced, the brighter the GFP signal will be.

 Figure 1. Hippocampal neuron stained with GFP for MAP2. Actin filaments are in red and DNA is in blue. The protein tags allow researchers to see MAP2 in the neuron’s long projections, or dendrites, and see that actin localizes in clumps along the dendrites at structures called spines. The DNA stain shows the cell’s nucleus.   Image via Halpain lab at UCSD  .

Figure 1. Hippocampal neuron stained with GFP for MAP2. Actin filaments are in red and DNA is in blue. The protein tags allow researchers to see MAP2 in the neuron’s long projections, or dendrites, and see that actin localizes in clumps along the dendrites at structures called spines. The DNA stain shows the cell’s nucleus. Image via Halpain lab at UCSD.

With a GFP tag, researchers can also see where the protein is made, where it is transported, and under what conditions this changes. Previously cells had to be fixed, killing them and freezing them in time to view their components. One of the most compelling aspects of fused fluorescent tags is their ability to be viewed in real time within living cells. Live imaging allows researchers to manipulate the cells and understand how different environmental changes can affect cellular components over time. By using different colors of fluorescent proteins to label different cellular proteins and looking at where and when they overlap with one another, researchers can look within the cell to understand how different proteins might interact.

Within each tagged cell is a constellation of glowing proteins, like the glow-in-the-dark stars stuck to a childhood ceiling. These cellular constellations move and interact in breathtaking ways that scientists are only beginning to understand, thanks to the discovery of GFP.