FTIR spectroscopy of the complex between pharaonis phoborhodopsin and its transducer protein

pharaonis phoborhodopsin (ppR; also called pharaonis sensory rhodopsin II, psRII) is a photoreceptor for negative phototaxis in Natronobacterium pharaonis. ppR activates the cognate transducer protein, pHtrII, upon absorption of light. ppR and pHtrII form a tight 2:2 complex in the unphotolyzed state, and the interaction is somehow altered during the photocycle of ppR. In this paper, we studied the influence of pHtrII on the structural changes occurring upon retinal photoisomerization in ppR by means of low-temperature FTIR spectroscopy. We trapped the K intermediate at 77 K and compared the ppR_K minus ppR spectra in the absence and presence of pHtrII. There are no differences in the X-D stretching vibrations (2700-1900 cm^-1) caused by presence of pHtrII. This result indicates that the hydrogen-bonding network in the Schiff base region is not altered by interaction with pHtrII, which is consistent with the same absorption spectrum of ppR with or without pHtrII. In contrast, the ppR_K minus ppR infrared difference spectra are clearly influenced by the presence of pHtrII in amide-I (1680-1640 cm^-1) and amide-A (3350-3250 cm^-1) vibrations. The identical spectra for the complex of the unlabeled ppR and ¹³C- or ¹⁵N-labeled pHtrII indicate that the observed structural changes for the peptide backbone originate from ppR only and are altered by retinal photoisomerization. The changes do not come from pHtrII, implying that the light signal is not transmitted to pHtrII in ppR_K. In addition, we observed D₂O-insensitive bands at 3479 (-)/3369 (+) cm^-1 only in the presence of pHtrII, which presumably originate from an X-H stretch of an amino acid side chain inside the protein.