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Green Fluorescent Protein Glows Gold

2008; Cell Press; Volume: 135; Issue: 6 Linguagem: Inglês

10.1016/j.cell.2008.11.025

1097-4172

Atsushi Miyawaki,

Photoreceptor and optogenetics research

The awarding of this year's Nobel Prize in Chemistry to Osamu Shimomura, Martin Chalfie, and Roger Tsien for their discovery and development of green fluorescent protein earns this humble jellyfish protein a place of honor in the biology research hall of fame. The awarding of this year's Nobel Prize in Chemistry to Osamu Shimomura, Martin Chalfie, and Roger Tsien for their discovery and development of green fluorescent protein earns this humble jellyfish protein a place of honor in the biology research hall of fame. This year, the Royal Swedish Academy of Sciences has awarded the Nobel Prize in Chemistry jointly to Osamu Shimomura of the Woods Hole Marine Biological Laboratory, Martin Chalfie of Columbia University, and Roger Y. Tsien of the University of California, San Diego (Figure 1). The Prize has been awarded for their discovery and development of green fluorescent protein (GFP)—first identified in the jellyfish Aequorea victoria—that has become one of the most important tools used in contemporary biology research. Although many molecules involved in biological systems have been identified and the hierarchy among these molecules has been elucidated, further understanding will require knowledge of how each component of the system is controlled in space and time. In the post-genomics era, researchers need a tool that enables the direct visualization of biological functions, and GFP has turned out to be that tool. GFP has a particularly unique history and one that has benefitted from the work of each of the three Nobel Laureates. From the discovery of the protein itself by Osamu Shimomura and the first expression studies by Martin Chalfie to the development of GFP and its many variants as ubiquitous laboratory tools by Roger Y. Tsien, it is both instructive and inspirational to review the life experiences and achievements of these three remarkable scientists. In 1961, newly arrived from Nagoya University, Japan, Osamu Shimomura began studying bioluminescence in the jellyfish Aequorea victoria with Frank H. Johnson at the University of Washington's Friday Harbor Laboratory, located on a small island near Victoria, British Columbia, Canada (Figure 2). The jellyfish displays a bright ring of green luminescence along the edge of its umbrella upon chemical or mechanical stimulation (Figure 2, inset). Every summer, from 1961 through the early 1990s, huge numbers of Aequorea victoria were skimmed from the sea by Friday Harbor researchers wielding long-handled nets. Initially, the precious jellyfish umbrella margins were removed with scissors; later on, harvesters adopted a specialized device (Figure 2) called a "jellyfish cutter." By the early 21st century, the species was virtually absent locally from the pelagic ecosystem, although this is probably due to environmental changes rather than zealous overcollection by eager scientists. In the 1960s, Shimomura identified and characterized the luminescent and fluorescent molecules responsible for Aequorea's bioluminescence (Shimomura, 1998Shimomura O. The Discovery of Green Fluorescent Protein. Green Fluorescent Protein: Properties, Applications, and Protocols.in: Chalfie M. Kain S. Wiley-Liss, Inc., Wilmington, DE1998Google Scholar). The luminescent molecule is the calcium-binding protein aequorin, which produces blue light in the presence of calcium ions, and Shimomura successfully extracted aequorin from the light organs of the jellyfish (Shimomura et al., 1962Shimomura O. Johnson F.H. Saiga Y. J. Cell. Comp. Physiol. 1962; 59: 223-239Crossref PubMed Scopus (1517) Google Scholar). However, structural determination of the aequorin luminophore proved to be a formidable task due to the fact that every treatment of this protein induced an intramolecular reaction that triggered the self-destruction of the luminophore. After thousands of attempts, Shimomura finally determined the structure of aequorin. In his own description of this project, he emphasizes that the work was greatly facilitated by the similarity of aequorin to the Cypridina luminescence system he had previously studied in the 1950s at Nagoya University. But what about the green luminescence of Aequorea victoria, which clearly could not be explained by aequorin alone. While purifying aequorin, Shimomura also isolated a "green protein" that exhibited conspicuously bright fluorescence (Shimomura et al., 1962Shimomura O. Johnson F.H. Saiga Y. J. Cell. Comp. Physiol. 1962; 59: 223-239Crossref PubMed Scopus (1517) Google Scholar). This "green protein" converted blue light to green light through an energy transfer process that does not involve radiation. In 1969, the protein was named "green fluorescent protein" (Hastings and Morin, 1969Hastings J.W. Morin J.G. Biol. Bull. 1969; 137: 402Google Scholar). With the aid of a luciferin compound from Cypridina that he had synthesized at Nagoya University, Shimomura elucidated the structure of the GFP chromophore and showed that it was p-hydroxybenzylideneimidazolinone (Shimomura, 1979Shimomura O. FEBS Lett. 1979; 104: 220-222Crossref Scopus (248) Google Scholar). In 2004, the Friday Harbor Laboratory celebrated its centenary. An international symposium entitled "Calcium-regulated photoproteins and green fluorescent proteins" was organized by William W. Ward to honor Shimomura. Although the symposium was relatively small, it brought together the three scientists who discovered, expressed, and then developed GFP: Shimomura, Chalfie, and Tsien. At that time I had a chance to visit Dr. Shimomura and his wife and to share their reminiscences of their long, colorful, and at times very challenging lives (I learned, for example, that they were both survivors of the atomic bombing of Nagasaki). Over the course of the symposium, the attendees revisited the decades-long history of the study of Aequorea victoria bioluminescence, from its humble beginnings with hand-held nets and scissors to its use as the marker of choice for monitoring gene expression and protein localization in most biology laboratories. Hearing these stories, I saw in a fresh light the importance of continuity in science. Martin Chalfie, an expert in the neurobiology of mechanoreceptors, had isolated many genes regulating the development of touch-sensitive cells in the roundworm Caenorhabditis elegans, a well-established and powerful genetic system (O'Hagan and Chalfie, 2006O'Hagan R. Chalfie M. Int. Rev. Neurobiol. 2006; 69: 169-203Crossref PubMed Scopus (31) Google Scholar). He had long dreamed of using fluorescent markers to visualize gene expression and protein localization in this tiny, optically transparent creature. In early 1989, Chalfie attended a seminar at Columbia University on bioluminescent organisms; there, he learned about GFP, which had been identified in and purified from the jellyfish by Shimomura. Enthusiastic about the potential of GFP, Chalfie set out to realize his dream, working in parallel with Douglas Prasher, then at Woods Hole. At times, effective communication between the two scientists was prevented by unfortunate circumstances. For example, immediately after he successfully identified the gene encoding GFP, Prasher called Chalfie at Columbia University. Unfortunately, at that time, Chalfie was at the University of Utah on sabbatical and could not be reached. Later on, in September 1992, during a discussion with a graduate student, a routine search of the latest literature on GFP alerted Chalfie to Prasher's article on the cloning of the gfp gene (Prasher et al., 1992Prasher D.C. Eckenrode V.K. Ward W.W. Prendergast F.G. Cormier M.J. Gene. 1992; 111: 229-233Crossref PubMed Scopus (1684) Google Scholar). He promptly contacted Prasher and asked him for the gfp clone. Following the publication of Prasher's study, several researchers attempted to heterologously express gfp and to observe luminescence in various model organisms, but all to no avail. Indeed, based on past experience, the biologists most familiar with fluorescent proteins were skeptical about such attempts. For example, phycobiliprotein extracted from seaweed emits strong fluorescence, yet the gene encoding this protein does not emit light once introduced into the cells of other organisms because emission requires the presence of algal-derived tetrapyrrole chromophores that are covalently bound to the cysteine residue of phycobiliprotein by a thioether bridge. According to the prevailing view, the formation of the GFP chromophore would likewise require one or more factors intrinsic to Aequorea victoria. Fortune, however, smiled on Chalfie. As it turned out, the gfp cDNA isolated by Prasher contained a sequence at one end that inhibited its expression. By eliminating the inhibitory sequence, Chalfie and his colleagues were able to successfully produce fluorescent bacteria and worms. Their research proved that GFP in fact encoded its own chromophore within its structure. Glowing a brilliant green, C. elegans expressing gfp under the control of the tubulin gene promoter graced the cover of the February 11, 1994 issue of Science (Chalfie et al., 1994Chalfie M. Tu Y. Euskirchen G. Ward W.W. Prasher D.C. Science. 1994; 263: 802-805Crossref PubMed Scopus (5283) Google Scholar). Roger Y. Tsien is more than a mere imaging specialist, he is a pioneer of fluorescence imaging tools and technologies. Renowned for developing calcium ion indicators (such as quin-2, fura-2, indo-1, fluo-3, and others) and their membrane-permeable derivatives in the 1980s, he was also involved in the generation of caged calcium ion compounds. Using fura-2 and indo-1, he established a method for quantitative measurements of intracellular calcium ion concentrations based on ratiometric imaging. In addition to calcium ions, he created fluorescent indicators for cAMP, pH, and membrane voltage. These techniques possessed tremendous potential for enhancing our understanding of many different facets of cell biology (Tsien, 1994Tsien R.Y. Chem. Eng. News. 1994; July 18: 34-44Crossref Scopus (56) Google Scholar). Tsien's work did not end with the development of indicators; indeed his achievements extended much further, into the fields of microscopic optical systems and analytical software development. Originally trained in synthetic organic chemistry, he has devoted his career to creating new fields in science. Indeed, his accomplishments attest to the fact that he has energetically pursued interdisciplinary studies rather than remaining focused on one specific area. It is no wonder that the many and varied interests of Roger Tsien drew him to GFP, one of nature's masterpieces. Using a recombinant GFP provided by Prasher, he elucidated the relationship between the protein's structure and its function. In particular, he conducted an in-depth study of how the conjugated double-bonded structure of the p-hydroxybenzylideneimidazolinone chromophore of GFP is formed and also examined the physico-chemical environment of the chromophore. Moreover, by introducing mutations into the gfp gene, he developed many useful GFP variants. Not all of these mutations were motivated by preceding biophysical studies. Indeed, it was through random mutagenesis that Tsien discovered the now commonly used enhanced GFP that contains a Ser65Thr substitution, as well as spectral GFP variants (Heim et al., 1994Heim R. Prasher D.C. Tsien R.Y. Proc. Natl. Acad. Sci. USA. 1994; 91: 12501-12504Crossref PubMed Scopus (1432) Google Scholar, Heim et al., 1995Heim R. Cubitt A.B. Tsien R.Y. Nature. 1995; 373: 663-664Crossref PubMed Scopus (1479) Google Scholar, Heim and Tsien, 1996Heim R. Tsien R.Y. Curr. Biol. 1996; 6: 178-182Abstract Full Text Full Text PDF PubMed Scopus (1141) Google Scholar) such as blue fluorescent protein (BFP) and cyan fluorescent protein (CFP). On the basis of GFP structural data, however, he designed and generated yellow fluorescent protein (YFP) with James Remington (Ormö et al., 1996Ormö M. Cubitt A.B. Kallio K. Gross L.A. Tsien R.Y. Reminton S.J. Science. 1996; 273: 1392-1395Crossref PubMed Scopus (1821) Google Scholar). Furthermore, the general principles discovered in studies of GFP turned out to be applicable to a growing number of newly discovered fluorescent proteins obtained by cloning genes from nonbioluminescent cnidarians. The review on GFP that Tsien published in 1998 has been cited in myriad scientific articles, and is widely regarded as a bible for biologists (Tsien, 1998Tsien R.Y. Annu. Rev. Biochem. 1998; 67: 509-544Crossref PubMed Scopus (4684) Google Scholar). The availability of numerous GFP variants has led research in the post-genomic era to even greater heights of multicolor imaging, allowing for the refinement and further development of techniques such as fluorescence resonance energy transfer (FRET), fluorescence cross correlation spectroscopy (FCCS), and bimolecular fluorescence complementation (BiFC). Concurrently, the work has motivated researchers across the globe to develop new genetically encoded fluorescent probes (Miyawaki, 2003Miyawaki A. Dev. Cell. 2003; 4: 295-305Abstract Full Text Full Text PDF PubMed Scopus (415) Google Scholar). Indeed, Tsien has produced a wide range of genetically encoded probes himself in order to aid in the exploration of new frontiers in biology research. I had the privilege to conduct research on GFP in Dr. Tsien's laboratory from 1995 to 1998. I also participated in a project involving the simulation of fluorescence profiles of GFP variants based on chromophore π-electron behaviors. Discussions with Roger often filled me with a sense of awe: here was a person who knew, inside-out, how electrons behave. Osamu Shimomura has always been motivated by a desire to understand the mechanisms of bioluminescence. Because the principal player in Aequorea bioluminescence is the chemiluminescent protein aequorin, Shimomura has devoted much of his life to the chemistry of this protein, culminating in a Nature paper published in 2000, where he reported the determination of its crystal structure (Head et al., 2000Head J.F. Inouye S. Teranishi K. Shimomura O. Nature. 2000; 405: 372-376Crossref PubMed Scopus (251) Google Scholar). As a result, he may have been a bit perplexed by the fact that the announcement of the 2008 Nobel Prize in Chemistry made no mention of aequorin. By contrast, GFP is a supporting player in bioluminescence of the jellyfish. After elucidating GFP's chromophore structure in 1979 (Shimomura, 1979Shimomura O. FEBS Lett. 1979; 104: 220-222Crossref Scopus (248) Google Scholar), Shimomura ended his research on GFP, and he has not been involved in the development of practical applications of the protein. Until the 2004 centenary celebration at Friday Harbor, he participated in none of the international symposia on GFP yet readily acknowledges the enormous usefulness of GFP to the biomedical sciences. For each of these three Nobel Laureates, it must be emphasized that their work on GFP has constituted only one part of their scientific careers. Each of them has made tremendous contributions in other lines of research: Shimomura in the field of chemiluminescence, Chalfie in the study of touch receptors, and Tsien in the development of chemical probes. In celebrating their achievement, we must also appreciate their broader work in parallel with their contributions to developing GFP technology. These three Nobel Laureates each played a unique part in the history of GFP. If they share one common characteristic, it is certainly that they are in awe of nature. Shimomura caught more than 850,000 jellyfish in order to purify and characterize GFP and consequently elucidated how Aequorea glows. Through heterologous expression of the gfp gene, Chalfie created glimmering E. coli bacteria and nematodes and found in GFP an extremely illuminating gift from nature. Tsien developed a plethora of useful GFP variants, which certainly outnumber the variants that evolution will produce over the next few thousand years. Tsien is in the habit of reminding us that nature has been kind. Perhaps only a scientist with his respect and reverence for nature would have been permitted to surpass it. I would like to thank K. Hong and D. Mou for helpful comments.