Diversity and Relationships Among the GFP Family

Schematic representation showing the main polypeptide chain forming the 11 strand [3-sheet barrel and central a-helix (grey), the chromophore buried in the barrel s interior (green), and the two highly conserved residues Arg 96 (purple, on the left) and Glu 222 (purple, on the right). 

Image built upon coordinates from the 1EMB entry of the Protein Data Bank [84] 

The unicity of the GFP family is better appreciated when knowing that all red GFPs mature from a green precursor carrying the same chromophore as AvGFP, to which they can eventually revert back [33, 41], while initially green GFPs can evolve in different ways toward red emission [20, 42-44], Similarly, many chromoproteins can be turned fluorescent at alkaline pHs [45], upon photoactivation [46], or 

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