Green fluorescent protein, GFP <p>The green fluorescent protein (GFP) is found in the jellyfish (<taxon tax_id="6100">Aequorea victoria</taxon>), and functions as an energy-transfer acceptor. It fluoresces <i>in vivo</i> upon receiving energy from the Ca²⁺-activated photoprotein aequorin. The protein absorbs light maximally at 395 nm and exhibits a smaller absorbance peak at 470 nm. The fluorescence emission spectrum peaks at 509 nm with a shoulder at 540 nm. The protein is produced in the photocytes and contains a chromophore, which is composed of modified amino acid residues. The chromophore is formed upon cyclisation of the residues ser-dehydrotyr-gly. There are several other members of the GFP family, which are able to fluoresce different colours, sveral of which are non-fluorescent [<cite idref="PUB00020645"/>]. These proteins are all essentailly encoded by single genes, since both the substrate and the catalytic enzyme for pigment biosynthesis are provided within a single polypeptide chain [<cite idref="PUB00020644"/>].</p><p>More information about this protein can be found at Protein of the Month: Green Fluorescent Protein [<cite idref="PUB00020721"/>].</p>