There are many GFP-like proteins that, despite being in the same protein family as GFP, are not directly derived from Aequorea victoria. These include dsRed, eqFP611, Dronpa, TagRFPs, KFP, EosFP/IrisFP, Dendra, and so on. Having been developed from proteins in different organisms, these proteins can sometimes display unanticipated approaches to chromophore formation. Some of these, such as KFP, are developed from naturally non- or weakly-fluorescent proteins to be greatly improved upon by mutagenesis. When GFP-like barrels of different spectra characteristics are used, the excitation spectra of one chromophore can be used to power another chromophore (FRET), allowing for conversion between wavelengths of light.
FMN-binding fluorescent proteins (FbFPs) were developed in 2007 and are a class of small (11–16 kDa), oxygen-independent fluorescent proteins that are derived from blue-light receptors. They are intended especially for the use under anaerobic or hypoxic conditions, since the formation and binding of the Flavin chromophore does not require molecular oxygen, as it is the case with the synthesis of the GFP chromophore.
Fluorescent proteins with other chromophores, such as UnaG with bilirubin, can display unique properties like red-shifted emission above 600 nm or photoconversion from a green-emitting state to a red-emitting state. They can have excitation and emission wavelengths far enough apart to achieve conversion between red and green light.
A new class of fluorescent protein was evolved from a cyanobacterial (Trichodesmium erythraeum) phycobiliprotein, α-allophycocyanin, and named small ultra red fluorescent protein (smURFP) in 2016. smURFP autocatalytically self-incorporates the chromophore biliverdin without the need of an external protein, known as a lyase. Jellyfish- and coral-derived GFP-like proteins require oxygen and produce a stoichiometric amount of hydrogen peroxide upon chromophore formation. smURFP does not require oxygen or produce hydrogen peroxide and uses the chromophore, biliverdin. smURFP has a large extinction coefficient (180,000 M cm) and has a modest quantum yield (0.20), which makes it comparable biophysical brightness to eGFP and ~2-fold brighter than most red or far-red fluorescent proteins derived from coral. smURFP spectral properties are similar to the organic dye Cy5.
Reviews on new classes of fluorescent proteins and applications can be found in the cited reviews.