ИСТИНА

iRFP713 belongs to near-infrared (NIR) fluorescent proteins (FPs) engineered from bacterial phytochromes. NIR absorbance and fluorescence of NIR FPs make them beneficial probes for in vivo imaging. Spectral properties of NIR FPs are attributed to their biliverdin chromophore (BV) enzymatically derived from heme. Covalent binding of BV to NIR FPs affects their stability and folding while the place of BV attachment influences their spectral maxima. Because of the BV chromophore and an unusual knotted protein fold iRFP713 is an interesting object for folding studies. We have shown that the figure-of-eight knot in iRFP713 structure do not interfere with the protein refolding. The unfolding–refolding studies of iRFP713 denaturation induced by guanidine hydrochloride and guanidine thiocyanate (GTC) revealed a gap between the mid-point of denaturing transition, monitored by parameters of tryptophan fluorescence and chromophore fluorescence or far-UV CD. The gap was more pronounced in the presence of GTC. We corroborated these data by kinetic spectroscopic and steady-state gel-filtration studies. We have revealed two intermediate states and characterized them as native-like monomeric and aggregated states.