Fluorescent proteins from the oceans: marine macromolecules as advanced imaging tools for biomedical research

Fluorescent proteins (FPs) of the green fluorescent protein (GFP)-like protein family are not only responsible for the spectacular colors of some reef corals, but they can be also found in several other marine invertebrate taxa. The chromophore of these proteins resides in the center of an 11-stranded ?-barrel and is formed by autocatalytic reactions in the presence of oxygen. In some proteins, photochemical reactions are involved in the maturation of the chromophore. The variability of the chromophore structures and the interactions with the surrounding protein scaffold are responsible for the diverse emission colors, ranging from cyan to green and yellow to red. FPs develop their fluorescence also upon expression in recombinant cells which allows their use as genetically encoded markers. Application examples include the labeling of cells, cellular compartment or proteins, studies of gene activity, and sensor studies. Recently, photoactivatable FPs enabled live-cell imaging with a resolution beyond the diffraction barrier of optical microscopy. Nevertheless, before their potential as advanced markers can be fully exploited, FPs often need to undergo extensive protein engineering to alter some detrimental properties such as the tendency to form stable multimers