Photochemistry and photoinduced proton-transfer by pharaonis phoborhodopsin

Phoborhodopsin (pR or sensory rhodopsin II, sRII) is a photoreceptor of the negative phototaxis of Halobacterium salinarum, and pharaonis phoborhodopsin (ppR or pharaonis sensory rhodopsin II, psRII) is a corresponding protein of Natronobacterium pharaonis. The photocycle of ppR is essentially as follows: ppR(498) → ppR_K(∼540) → ppR_KL(512) → ppR_L(488) → ppR_M(390) → ppR_o(560) → ppR (numbers in parenthesis denote the maximum absorbance). The photocycle is very similar to that of bacteriorhodopsin, but the rate of initial pigment recovery is about two-orders of magnitude slower. By low-temperature spectroscopy, two K-intermediates were found but the L intermediate was not detected. The lack of L indicates extraordinary stability of K at low temperature, ppR_M is photoactive similar to M of bR. The ground state ppR contains only all-trans retinal whereas ppR_M and ppR_o contain 13-cis and all-trans, respectively. ppR has the ability of light-induced proton transport from the inside to the outside. Proton uptake occurs at the formation of ppR_o and the release at its decay. ppR associates with its transducer and this complex transmits a signal to the cytoplasm. The proton transport ability is lost when the complex forms, but the proton uptake and release still occur, suggesting that the proton movement is non-electrogenic (release and uptake occur from the same side). The stoichiometry of the complex between ppR and the transducer is 1 : 1. ppR or pR has absorption maximum at ∼500 nm, which is blue-shifted from those of other archaeal rhodopsins. The molecular mechanism of this color regulation is not yet solved.