So it was green fluorescent protein after all! We can argue about whether this was a pure chemistry prize or another quasi-biology one, but either way, the award is a strong one. So, what is the stuff and what’s it do?
Osamu Shimomura discovered the actual protein back in 1962, isolating it from the jellyfish Aequoria victoria. These were known to be luminescent creatures, but when the light-emitting protein was found (named aequorin), it turned out to give off blue light. That was strange, since the jellyfish were known for their green color. Shimomura then isolated another protein from the same jellyfish cells, which turned out to absorb the blue light from aequorin very efficiently and then fluoresce in the green: green fluorescent protein. The two proteins are a coupled system, an excellent example of a phenomenon known as FRET (fluorescence resonance energy transfer), which has been engineered into many other useful applications over the years.
Fluorescence is much more common in inorganic salts and small organic molecules, and at first it was a puzzle how a protein could emit light in the same way. As it turns out, there’s a three-amino-acid sequence right in the middle of its structure (serine-tyrosine-glycine) that condenses with itself when the protein is folded properly and makes a new fluorescent species. (The last step of the process is reaction with ambient oxygen). The protein has a very pronounced barrel shape to it, and lines up these key amino acids in just the orientation needed for the reaction to go at a reasonable rate (on a time scale of tens of minutes at room temperature). This is well worked out now, but it was definitely not obvious at the time.
In the late 1980s, for example, the gene for GFP was cloned by Doug Prasher, but he and his co-workers believed that they could well express a non-fluorescent protein that would need activation by some other system. He had the idea that this could be used as a tag for other proteins, but was never able to get to the point of demonstrating it, and will join the list of people who were on the trail of a Nobel discovery but never quite got there. Update: Here's what Prasher is doing now - this is a hard-luck story if I've ever heard one Prasher furnished some of the clone to Martin Chalfie at Columbia, who got it to express in E. coli and found that the bacteria indeed glowed bright green. (Other groups were trying the same thing, but the expression was a bit tricky at the time). The next step was to express it in the roundworm C. elegans (naturally enough, since Chalfie had worked with Sydney Brenner). Splicing it in behind a specific promoter caused the GFP to express in definite patterns in the worms, just as expected. This all suggested that the protein was fluorescing on its own, and could do the same in all sorts of organisms under all sorts of conditions.
And so it’s proved. GFP is wonderful stuff for marking proteins in living systems. Its sequence can be fused on to many other proteins without disturbing their function, it folds up just fine with no help to its active form, and it’s bright and very photoefficient. Where Roger Tsien enters the picture is in extending this idea to a whole family of proteins. Tsien worked out the last details of the fluorescent structure, showing that oxygen is needed for the last step. He and his group then set out to make mutant forms of the protein, changing the color of its fluorescence and other properties. He’s done the same thing with a red fluorescent protein from coral, and this work (which continues in labs all over the world) has led to a wide variety of in vivo fluorescent tags, which can be made to perform a huge number of useful tricks. They can sense calcium levels or the presence of various metabolites, fluoresce only when they come into contact with another specifically labeled protein, used in various time-resolved techniques to monitor the speed of protein trafficking, and who knows what else. A lot of what we’ve learned in the last fifteen years about the behavior of real proteins in living cells has come out of this work – the prize is well deserved.
I want to close with a bit of an interview with Martin Chalfie, which is an excellent insight into how things like this get discovered (or don't!)
Considering how significant GFP has been, why do you think no one else came up with it, while you were waiting for Doug Prasher to clone it?
"That’s a very important point. In hindsight, you wonder why 50 billion people weren’t working on this. But I think the field of bioluminescence or, in general, the research done on organisms and biological problems that have no immediate medical implications, was not viewed as being important science. People were working on this, but it was slow and tedious work, and getting enough protein from jellyfish required rather long hours at the lab. They had to devise ways of isolating the cells that were bioluminescent and then grinding them up and doing the extraction on them. It’s not like ordering a bunch of mice and getting livers out and doing an experiment. It was all rather arduous. It’s quite remarkable that it was done at all. It was mostly biochemists doing it, and they were not getting a lot of support. In fact, as I remember it, Doug Prasher had some funding initially from the American Cancer Society, and when that dried up he could not get grants to pursue the work. I never applied for a grant to do the original GFP research. Granting agencies would have wanted to see preliminary data and the work was outside my main research program. GFP is really an example of something very useful coming from a far-outside-the-mainstream source. And because this was coming from a non-model-organism system, these jellyfish found off the west coast of the U.S., people were not jumping at the chance to go out and isolate RNAs and make cDNAs from them. So we’re not talking about a field that was highly populated. It was not something that was widely talked about. At the time, there was a lot of excitement about molecular biology, but this was biochemistry. The discovery really was somewhat orthogonal to the mainstream of biological research."
Here's an entire site dedicated to the GFP story, full of illustrations and details. That interview with Chalfie is here, with some background on his part in the discovery. Science background from the Nobel Foundation is here (PDF), for those who want even more).