Green Fluorescent Protein
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Phylogeny of GFP-like proteins Courtesy of Mike Matz |
Divergence
The colour differences among GFP-like proteins arise from changes in the structure of the chromophore, with variations within each colour class as well. These changes result from altered catalytic mechanisms. The chromophore is formed by the tripeptide Ser65-Tyr66-Gly67 in A. victoria, which is modified to form a five-member ring. From the corresponding residues in the other sequenced proteins given in the Table, it is clear that Tyr66 and Gly67 are highly conserved, while Ser65 is variable. The wide variations in colour that arise from changes in enzymatic properties appear to be the result of minimal alterations to protein sequence and structure. As such, there appears to have been multiple events of colour diversification, with the possibility that new colours may still be arising.
Why so many colours?
The question arising when one sees the myriad of colours in coral reefs is: is this diversity reflected in the function of these proteins or is it random variation? So far the function of GFP-related colouration remains elusive. In general, bioluminescence in sea creatures is related to illuminating areas to find food, signalling for a possible mate, emitting clouds of illuminated material for self-defence, or mimicking sparkling sunlight to hide their shadow from predators lurking below. There is evidence that jellyfish may use bioluminescence for defence; some species of jellyfish release clouds of bioluminescence into the surrounding water to blindingly distract predators. The localisation of bioluminescence in jellyfish is species-specific, raising the possibility that it could also function in finding a mate.
Non-bioluminescent colouration could have different functional roles. Coral fluorescence may function in photoprotection by scattering light, or by converting it into less harmful wavelengths. It could also serve to enhance photosynthesis in low-light conditions by supplying light of the correct wavelength to zooxanthellae, symbiotic algae that supply more than 90% of the coral’s energy budget through photosynthesis. The function of the non-fluorescent chromoproteins remains the most elusive. Chromoproteins often occur alongside fluorescent proteins and can sometimes be found in the same physiological distribution in different genera (e.g. tentacle tips), indicating a distinctive functional role for these proteins. Specialised functions would then support the natural selection of colour diversity, which appears to arise easily with few sequence changes.
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