pHtrII, a pharaonis halobacterial transducer protein, possesses two transmembrane helices and forms a signaling complex with pharaonis phoborhodopsin (ppR, also called pharaonis sensory rhodopsin II, NpSRII) within the halobacterial membrane. This complex transmits a light signal to the sensory system located in the cytoplasm. It has been suggested that the linker region connecting the transmembrane region and the methylation region of pHtrII is important for binding to ppR and subsequent photosignal transduction. In this study, we present evidence to suggest that the linker region itself interacts directly with ppR in addition to the interaction in the membrane region. An in vitro pull-down assay revealed that the linker region bound to ppR, and its dissociation constant (KD) was estimated to be approximately 10 μM using isothermal titration calorimetry (ITC). Solution NMR analyses showed that ppR interacted with the linker region of pHtrII (pHtrIIG83-Q149) and resulted in the broadening of many peaks, indicating structural changes within this region. These results suggest that the pHtrII linker region interacts directly with ppR. There was no demonstrable interaction between the C-terminal region of ppR (ppRGly224-His247) and either the linker region (pHtrIIG83-Q149) or the transmembrane region (pHtrII M1-E114) of pHtrII. On the basis of the NMR, CD, and photochemical data, we discuss the structural changes and role of the linker region of pHtrII in relation to photosignal transduction.
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