Nanoscale Comparison Of Light And Dark-Adapted States Of Bacterial Photoreceptors

Bacteriophytochromes, (Bphs), are a signaling proteins from bacteria that regulate response to red light and are vital to the survival and function of a cell. Bphs contain a photosensory module covalently linked to an output domain, which is typically histidine kinase (HK). They undergo reversible photoconversion that prompts a series of global conformational changes upon absorption of a photon.  Due to the protein size and light sensitivity, obtaining structures of the intact proteins through Nuclear Magnetic Resonance or X-ray crystallography has proven to be challenging. Therefore, molecular details about the light and dark-adapted structures of the intact Bphs are not well understood at the atomic level. In this work, scanning tunneling microscopy has been used to characterize RpBphP3 (P3) from the photosynthetic bacterium Rhodopseudomonas palustris. P3 is the only Bph that absorbs in the near-red form  (Pnr) upon photoexcitation. To our knowledge, this is the first characterization of the individual domains of an intact Bph in both the light and dark-adapted states with nanometer scale resolution.  Future goals include comparing the images of P3 to those of related protein from Stigmatella aurantiaca, SaBphP in both the dark-adapted state (Pr) and the light-adapted state (Pfr).  SaBphP undergoes limited photoconversion and lacks a conserved amino acid histidine that stabilizes biliverdin chromophore, necessary for photoconversion in the dark-adapted state.  This difference may lead to alternate structural changes upon exposure to light that could yield new insight into light sensing properties of Bphs and the mechanism of photoconversion.