1.8 angstrom bright-state structure of the reversibly switchable fluorescent protein Dronpa guides the generation of fast switching variants

RSFPs (reversibly switchable fluorescent proteins) may be repeatedly converted between a fluorescent and a non-fluorescent state by irradiation and have attracted widespread interest for many new applications. The RSFP Dronpa may be switched with blue light from a fluorescent state into a non-fluorescent state, and back again with UV light. To obtain insight into the underlying molecular mechanism of this switching, we have determined the crystal structure of the fluorescent equilibrium state of Dronpa. Its bicyclic chromophore is formed spontaneously from the Cys(62)Tyr-(63)Gly(64) tripeptide. In the fluorescent state, it adopts a slightly non-coplanar cis conformation within the interior of a typical GFP (green fluorescent protein) beta-can fold. Dronpa shares some structural features with asFP595, another RSFP whose chromophore has previously been demonstrated to undergo a cis-trans