All the colors of the rainbow

GFP was the first fluorescent protein to be dicovered, and subsequently used in biological research. However, by now, the biological community has found or developed an enormous number of fluorescent proteins of many colors.

According to my count (based on recent review papers)  there are over 90(!) differnt fluorescent proteins. These proteins can be classified based on several charactereisitcs:

Emission color: is the most obivous classification. The classification generally goes by: Blue (424-457nm), Cyan (474-492nm), Green (499-509nm), Yellow (524-529nm), Orange (559-565nm), Red (584-610nm) and Far-Red (625-650nm).

Bacteria expressing differnt FPs were plated to create a nice picture (source: Roger Tsien lab)

Oligomerization: many of the FPs are monomeric (i.e. fluorece as single molecules). Others may be dimeric (two) or tetrameric (four).

Photoactivation/photoconversion: some proteins can switch there color when activated by a specific excitation wavelength. This means that the emission wavelength can change from green to red, for instance. In a few cases, the initial state of the protein is non-fluorescent, thus allowing very low background level of fluorescence. This group can be sub-divided into reversible and non-reversible photoactivatable proteins.

Fluorescnet timers – These protein change their color over time. Therefore, these can be used as “timers” for cellular processes following their activation.

Large Stokes shift (LSS): Stokes shift (named after George G. Stokes) is the shift in wavelength from excitation to emission. For most FPs, Stokes shift is less than 50nm (usually much less).  For LSS proteins, the differnce is over 100nm (i.e. cells are excited by UV light or blue light and their emission is Green or Red light).

Natural vs. engineered: There is currently a lot of work invested in developing new colors and new activatable proteins by directed mutagenesis.

Three excellent review papers on the differnt kinds of FPs:

Stepanenko et. al. (2008) “Fluorescent proteins as biomarkers and Biosensors: Throwing color lights on molecular and cellular processes” Curr. Protein. Pept. Sci. 9(4):338.

Chudakov et. al. (2010) “Fluorescent proteins and their applications in imaging living cells and tissues”  Physiol. Rev. 90:1103.

Wu et. al. (2011) “Modern fluorescent proteins and imaging technologies to study gene expression, nuclear localization, and dynamics” Curr. Opin. Cell. Biol. 23:310.

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