Pigment protein that is yellow in color. Exhibits a smaller absorbance peak at 494 nm. Has a strong fluorescence emission spectrum which peaks at 538 nm. Homotetramer. Tentacle and oral disk. Contains a chromophore consisting of modified amino acid residues. The chromophore is formed by autocatalytic backbone condensation between Xaa-N and Gly-(N+2), and oxidation of Tyr-(N+1) to didehydrotyrosine. In addition, the residue N lysine undergoes cyclization. The alpha-amino nitrogen is replaced by the epsilon-amino nitrogen, the peptide chain is broken, residue N-1 is released as an amide, and a double bond is formed between the alpha-carbon and the nitrogen so that a tetrahydropyridine ring results. Maturation of the chromophore requires nothing other than molecular oxygen. Fluorescent proteins have become a useful and ubiquitous tool for making chimeric proteins, where they function as a fluorescent protein tag. Typically they tolerate N- and C-terminal fusion to a broad variety of proteins. They have been expressed in most known cell types and are used as a noninvasive fluorescent marker in living cells and organisms. They enable a wide range of applications where they have functioned as a cell lineage tracer, reporter of gene expression, or as a measure of protein-protein interactions. Fluorescence excitation of the Glu-66 mutant is at 493 nm and 550 nm with intense green emission at 405 nm and a weak red emission at 576 nm. Fluorescence emission of the Asp-66 mutant is at 524 nm and 552 nm and with a broad red emission shoulder extending from 650 nm. Belongs to the GFP family.