The chromophore responsible for the optical properties in GFP-like proteins is composed of X-Tyr-Gly and is able to form an imidazolinone structure through an autocyclization step, which involves the residues in the vicinity of the chromophore environment such as the highly conserved E215 and R95 in DsRed and the E222 and R96 in avGFP.
Source: wiktionary