KikGR is a fluorescent protein engineered to display green-to-red photoconvertibility that is induced by irradiation with ultraviolet or violet light. Similar to Kaede and EosFP, two naturally occurring photoconvertible proteins, KikGR contains a His62-Tyr63-Gly64 tripeptide sequence, which forms a green chromophore that can be photoconverted to a red one via formal β-elimination and subsequent extension of a π-conjugated system. Using a crystallizable variant of KikGR, we determined the structures of both the green and red state at 1.55 Å resolution. The double bond between His62-Cα and His62-Cβ in the red chromophore is in a cis configuration, indicating that rotation along the His62 Cα-Cβ bond occurs following cleavage of the His62 Nα-Cα bond. This structural rearrangement provides evidence that the β-elimination reaction governing the green-to-red photoconversion of KikGR follows an E1 (elimination, unimolecular) mechanism.
