Conformational partitioning in pH-induced fluorescence of the kindling fluorescent protein (KFP)

Kindling fluorescent protein (KFP) is considered as a prospective fluorophore for high-resolution nanoscopy. Analysis of pH dependence of the absorption and fluorescence spectra of KFP in aqueous solutions prompted us to assume that a shift in conformational equilibrium is responsible for substantial enhancement of red fluorescence in KFP at alkaline pH. Variations in pH also resulted in noticeable shifts in band maxima for absorption, fluorescence excitation, and fluorescence emission. These observations can be interpreted as an appearance of pH-induced fluorescent conformational states of the protein. On the basis of the available crystal structures of the protein and the results of molecular modeling, we suggest that appearance of these pH-induced fluorescent states is due to changes in the hydrogen bond network around the chromophore moiety (but not the cromophore itself), especially those associated with the side chains of Cys62 and Ser158. We hypothesize that conformational partitioning and fluctuations in protein ionization at alkali pH play an essential role in the appearance of fluorescent properties of KFP.