Discovery and Major Milestones.
Green Fluorescent Protein was discovered by Shimomura et al (1) as a companion protein to aequorin, the famous chemiluminescent protein from Aequorea jellyfish. In a footnote to their account of aequorin purification, they noted that “a protein giving solutions that look slightly greenish in sunlight through only yellowish under tungsten lights, and exhibiting a very bright, greenish fluorescence in the ultraviolet of a Mineralite, has also been isolated from squeezates.” This description of the appearance of GFP solutions is still accurate. The same group (2) soon published the emission spectrum of GFP, which peaked at 508 nm. They noted that the green bioluminescence of living Aequorea tissue also peaked near this wavelength, whereas the chemiluminescence of pure aequorin was blue and peaked near 470 nm, which was close to one of the excitation peaks of GFP. Therefore the GFP converted the blue emission of aequorin to the green glow of the intact cells and animals. Morin & Hastings (3) found the same color shift in the related coelenterates Obelia (a hydroid) and Renilla (a sea pansy) and were the first to suggest radiationless energy transfer as the mechanism for exciting coelenterate GFPs in vivo. Moriseetal(4)purifiedandcrystallizedGFP,measureditsabsorbancespectrum and fluorescence quantum yield, and showed that aequorin could efficiently transfer its luminescence energy to GFP when the two were coadsorbed onto a cationicsupport. Prendergast&Mann(5)obtained the first clear estimate for the
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monomer molecular weight. Shimomura (6) proteolyzed denatured GFP, analyzed the peptide that retained visible absorbance, and correctly proposed that the chromophore is a 4-(p-hydroxybenzylidene)imidazolidin-5-one attached to the peptide backbone through the 1- and 2-positions of the ring. Aequorea and Renilla GFPs were later shown to have the same chromophore (7); and the pH sensitivity, aggregation tendency (8), and renaturation (9) of Aequorea GFP were characterized. But the crucial breakthroughs came with the cloning of the gene by Prasher et al (10) and the demonstrations by Chalfie et al (11) and Inouye & Tsuji (12) that expression of the gene in other organisms creates fluorescence. Therefore the gene contains all the information necessary for the posttranslational synthesis of the chromophore, and no jellyfish-specific enzymes are needed.