FTIR Spectroscopy Revealing Light-Dependent Refolding of the Conserved Tongue Region of Bacteriophytochrome

Bacteriophytochromes (BphPs) constitute a class of photosensory proteins that toggle between Pr and Pfr functional states through absorption of red and far-red light. The photosensory core of BphPs is composed of PAS, GAF, and PHY domains. Here, we apply FTIR spectroscopy to investigate changes in the secondary structure of Rhodopseudomonas palustris BphP2 (RpBphP2) upon Pr to Pfr photoconversion. Our results indicate conversion from a beta-sheet to an alpha-helical element in the so-called tongue region of the PHY domain, consistent with recent X-ray structures of Deinococcus radiodurans DrBphP in dark and light states (Takala, H.; et al. Nature 2014, 509, 245-248). A conserved Asp in the GAF domain that noncovalently connects with the PHY domain and a conserved Pro in the tongue region of the PHY domain are essential for the beta-sheet-to-alpha-helix conversion.