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Fig. 1. (A) FP β-barrel architecture and approximate dimensions, and chromophore structures of common Aequorea FP derivatives. (1) BFP, (2) CFP, (3) EGFP, (4) YFP. The tryptophan residue (Trp66) in (2) is illustrated in the cis conformation as occurs for Cerulean derivatives (Malo et al., 2007) rather than the trans isomer that is common to CFP and related variants. Portions of the chromophores that are conjugated and give rise to fluorescence are shaded with colors corresponding to the emission spectral profile. (B) Aequorea GFP mutation map showing common mutations superimposed on a topological layout of the peptide structure. β-sheets are numbered and depicted as thin, green cylinders with an arrow pointing towards the C-terminus, whereas 
-helices are depicted as gray cylinders. Mutations are color-coded to represent the variants to which they apply: BFPs (blue), CFPs (cyan), GFPs (green), YFPs (yellow), Sapphire (violet), folding, shared and monomerizing (gray). Note that almost 75% of the mutations are located in the central helix and in β-sheet strands 7, 8 and 10. In general, wavelength-specific mutations occur near the central helix containing the chromophore, whereas folding mutations occur throughout the sequence. Many of the cyan and yellow FP mutations introduced near the termini of the proteins resulted during the CyPet and YPet mutagenesis efforts (Nguyen and Daugherty, 2005). Several of the sfGFP folding mutations (S30R, Y39N, F99S and N105T) also occur away from the chromophore. The monomerizing mutation A206K is useful for all known GFP derivatives, but is replaced by A206V in sfGFP and EBFP2.
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