Things that glow in the night

I'm presenting something a little different this week. As you might know, the Nobel prize in chemistry was awarded to a trio of scientists for their pioneering work in the field of fluorescent proteins. Osamu Shimomura, Roger Y. Tsien, and Martin Chalfie, who shared the prize, have defined the field for the last 50 years. Dr. Shimomura identified the first two members of the ever-expanding family of these molecules, which are called aequorin and the aptly named Green Fluorescent Protein (GFP). Dr. Chalfie performed early work with the GFP gene. Dr. Tsien is the current guru who has purified many more versions of these proteins, and also created many modified versions that fluoresce different colors.

You might find yourself asking, what exactly is a fluorescent protein? A fluorescent protein contains a special chemical structure called a 'chromophore', a fancy term for something that can absorb light (sometimes several colors), and then release, or 'emit', it as one specific color (for our purposes). For example, GFP can absorb light in the UV and visible blue portions of the electromagnetic spectrum, and then emit green light. The actual chemistry is a little more complicated than that, but I'm no chemist so that's the best you're going to get out of me.

Why are these proteins important? They are fantastic laboratory tools to help us locate and track other proteins we might be studying. With the discovery of DNA, the elucidation of genes and now the sequencing of the entire human genome, we now have the capability to combine the sequence of the gene we are studying with the sequence for a fluorescent protein. This creates a hybrid, or 'tagged' protein, that contains the protein we are studying with a nice little light-sensitive tag. We can use this hybrid to determine what type of organs or cells our protein is found in, and with advanced microscope techniques we can now even follow the movements of individual 'tagged' proteins inside a single cell. We do this by flashing the appropriate type of light into the cell and looking for the fluorescent response of the tag. Additionally, some of these fluorescent proteins like aequorin will only fluoresce in the presence of other molecules like calcium. This allows us to use them as indicators, or 'probes', for these molecules within cells. Also, we can use multiple, complimentary fluorescent proteins attached to two different proteins we might be studying to see if these two proteins interact by creating a chain reaction of light. All told, this is a powerful technique because it allows us to use a minimally invasive, genetically encoded system to study our genes of interest.

I thought I would provide a brief review of some of the important discoveries or techniques that hinged upon fluorescent proteins.

• ATP synthase movement [1997]. Researchers attached a long fluorescent protein to a protein called ATP synthase. ATP synthase is responsible for producing ATP, the main energy source used by the cells in our bodies. Think of it like a windmill producing renewable energy - not oil. In fact, the comparison is apt, because it turns out (no pun intended) that ATP rotates as it produces ATP. In fact, the synthase uses electrical energy in the cell to drive a 'crankshaft' that rotates a barrel-like portion on top that produces ATP. The researchers demonstrated this by literally taking very fast freeze-frame photos of the fluorescent marker that showed it turning around. This is one of those beautiful, elegant experiments that convinced me to pursue science as a career. I always find it amazing to think of microscopic proteins functioning as little machines. It really demonstrates that the fundamental laws of mechanics can operate on a minute scale.

• Brainbow [2007]. Yes, you read that correctly. From the Center of Brain Science at Harvard, featuring Josh Sanes and Jeff Lichtman, scientists created a rainbow in the brain. Imagine every cell in the brain fluorescing in a different color. Well, that's a slight exageration, but using combinations of genetically encoded fluorescent proteins the researchers were able to generate 90 colors in the mouse brain. Since there were so many colors, it's highly likely that adjacent neurons, or neurons connected to each other, will be different colors. This allows researchers to differentiate between different components of brain circuits when recording electrical currents or imaging brain activity. Beyond just looking cool, it could prove to be a very powerful tool to help us elucidate how individual neurons contribute to the functioning of brain circuits, and complex behavior.

The uses of fluorescent proteins are legion. As scientists, we use them almost every day. They have been critical tools for basic science and drug discovery for many years, and their uses are still expanding.