pharaonis phoborhodopsin (ppR, also called pharaonis sensory rhodopsin II, psRII) is a receptor for negative phototaxis in Natronobacterium pharaonis. It forms a 2:2 complex with its transducer protein, pHtrII, in membranes, and the association is weakened by 2 orders of magnitude in the M intermediate. Such change is believed to correspond to the transfer of the light signal to pHtrII. In this paper, we applied Fourier transform infrared (FTIR) spectroscopy to the active M intermediate in the absence and presence of pHtrII. The obtained difference FTIR spectra were surprisingly similar, notwithstanding the presence of pHtrII. This result strongly suggests that the transducer activation in the ppR-pHtrII system does not induce secondary structure alterations of the pHtrII itself. On the other hand, we found that the hydrogen bond of the OH group of Thr204 is altered in the primary K intermediate, but restored in the M intermediate. The hydrogen bond of Asn74 in pHtrII is strengthened in M, presumably because of the change in interaction with Tyr199 of ppR. These facts provided a light signaling pathway from Lys205 (retinal) of the receptor to Asn74 of the transducer through Thr204 and Tyr199. Transducer activation is likely to involve a relaxation of Thr204 in the receptor and hydrogen bonding alteration of Asn74 in the transducer, during which the helices of the transducer perform rigid-body motion without changing their secondary structures.
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