title: Spectroscopic evidence for the formation of an N intermediate during the photocycle of sensory rhodopsin II (phoborhodopsin) from Natronobacterium pharaonis.
abstract: Sensory rhodopsin II is a seven transmembrane helical retinal protein and functions as a photoreceptor protein in negative phototaxis of halophilic archaea. Sensory rhodopsin II from Natronomonas pharaonis (NpSRII) is stable under various conditions and can be expressed functionally in Escherichia coli cell membranes. Rhodopsins from microorganisms, known as microbial rhodopsins, exhibit a photocycle, and light irradiation of these molecules leads to a high-energy intermediate, which relaxes thermally to the original pigment after passing through several intermediates. For bacteriorhodopsin (BR), a light-driven proton pump, the photocycle is established as BR → K → L → M → N → O → BR. The photocycle of NpSRII is similar to that of BR except for N, i.e., M thermally decays into the O, and N has not been well characterized in the photocycle. Thus we here examined the second half of the photocycle in NpSRII, and in the present transient absorption study we found the formation of a new photointermediate whose absorption maximum is ∼500 nm. This intermediate becomes pronounced in the presence of azide, which accelerates the decay of M. Transient resonance Raman spectroscopy was further applied to demonstrate that this intermediate contains a 13-cis retinal protonated Schiff base. However, detailed analysis of the transient absorption data indicated that M-decay does not directly produce N but rather produces O that is in equilibrium with N. These observations allowed us to propose a structural model for a photocycle that involves N.
authors.affiliation: Department of Chemistry and Applied Chemistry, Graduate School of Science and Engineering, Saga University, Saga 840-8502, Japan.
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Tateishi
authors.forename: Takayuki
authors.initials: T
authors.lastname: Abe
authors.forename: Jun
authors.initials: J
authors.lastname: Tamogami
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Nakao
authors.forename: Takashi
authors.initials: T
authors.lastname: Kikukawa
authors.forename: Naoki
authors.initials: N
authors.lastname: Kamo
authors.forename: Masashi
authors.initials: M
authors.lastname: Unno
mesh_terms: D000042:Absorption; D001386:Azides / Q000494:pharmacology; D025602:Halorhodopsins / Q000737:chemistry* / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008027:Light; D019633:Natronobacterium / Q000378:metabolism* / Q000528:radiation effects; D025603:Sensory Rhodopsins / Q000737:chemistry* / Q000378:metabolism*; D013059:Spectrum Analysis, Raman / Q000379:methods*; D014732:Vibration
chemical_list: D001386:Azides; D025602:Halorhodopsins; D025603:Sensory Rhodopsins; C061508:sensory rhodopsin II protein, archaeal