This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. e propose continued structural studies on the basis of fluorescence activation of the 'kindling fluorescent protein' (KFP), a chromoprotein distantly related to the Green fluorescent Protein. KFP is initially non-fluorescent and undergoes a remarkable transition upon illumination with green light in the range of 520-580 nm - the protein becomes transiently fluorescent, a process termed 'kindling'. Light emission peaks in the orange-red at 595 nm. When illumination ceases, the fluorescent ('kindled') state decays to a nonfluorescent state with a time constant of about 100 sec or it can be rapidly quenched by illumination with blue light in the range of 450 nm. Multiple structures of KFP variants have been solved to date including dark-state, a putative light-activated state, and permanently fluorescent mutant structures. It has been proposed that chromophore trans-cis isomerization is the basis for the transition between fluorescent and nonfluorescent states. Our own light activation experiments with the A143G variant (KFP-1) have yet to reveal the presence of the cis chromophore. Rather, there is an apparent conformational shift in an adjacent histidine residue and a concomitant increase in trans chromophore planarity. Independent studies of a separate variant (A143S) showed that light illumination drives the formation of a cis species. However, the chromophore exhibits the same deviation from planarity found in the dark-state structures. This result is surprising given that multiple structures exist of both strongly fluorescent and weak fluorescent GFP homologues and in all cases the fluorescent proteins exhibit a high degree of chromophore planarity while the nonfluorescent proteins do not. We propose crystallographic studies of the dark-state and photoactivated A143S variant at room temperature and in frozen crystals in order to reveal the structural basis for activation phenomenon.
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