Photochemistry of a putative new class of sensory rhodopsin (SRIII) coded by xop2 of Haloarcular marismortui

Baliga et al. (2004) [1] reported the existence of a functionally unpredictable opsin gene, named xop2, in Haloarcula marismortui, a holophilic archaeon. Ihara et al. [38] performed molecular phylogenetic analysis and determined that the product of xop2 belonged to a new class of opsins in the sensory rhodopsins. This microbial rhodopsin was therefore named H. marismortui sensory rhodopsin III (HmSRIII). Here, we functionally expressed HmSRIII in Escherichia coli cell membranes to examine the photochemistry. The wavelength of maximum absorption ( λ max) for HmSRIII was 506 nm. We observed a very slow photocycle that completed in ∼50 s. Intermediates were defined as M ( λ max ∼ 380 nm), N ( λ max ∼ 460 nm) and O ( λ max ∼ 530 nm) 0.01 s after the flash excitation. The nomenclature for these intermediates was based on their locations along the absorption maxima of bacteriorhodopsin. Analysis of laser-flash-photolysis data in the presence and absence of azide gave the following results: (1) an equilibrium between N and O was attained, (2) the direct product of the M-decay was O but not N, and (3) the last photo-intermediate (HmSRIII′) had a λ max similar to that of the original, and its decay rate was very slow. Resonance Raman spectroscopy revealed that this N-intermediate had 13- cis retinal conformation. Proton uptake occurred during the course of M-decay, whereas proton release occurred during the course of O-decay (or exactly N–O equilibrium). Very weak proton-pumping activity was observed whose direction is the same as that of bacteriorhodopsin, a typical light-driven proton pump.