Photochemistry of a Dual-Bacteriorhodopsin System in Haloarcula marismortui: HmbRI and HmbRII

The Blue-Green Sensory Rhodopsin SRM from Haloarcula marismortui Attenuates Both Phototactic Responses Mediated by Sensory Rhodopsin I and II in Halobacterium salinarum

Haloarchaea utilize various microbial rhodopsins to harvest light energy or to mediate phototaxis in search of optimal environmental niches. To date, only the red light-sensing sensory rhodopsin I (SRI) and the blue light-sensing sensory rhodopsin II (SRII) have been shown to mediate positive and negative phototaxis, respectively. In this work, we demonstrated that a blue-green light-sensing (504 nm) sensory rhodopsin from Haloarcula marismortui, SRM, attenuated both positive and negative phototaxis through its sensing region. The H. marismortui genome encodes three sensory rhodopsins: SRI, SRII and SRM. Using spectroscopic assays, we first demonstrated the interaction between SRM and its cognate transducer, HtrM. We then transformed an SRM-HtrM fusion protein into Halobacterium salinarum, which contains only SRI and SRII, and observed that SRM-HtrM fusion protein decreased both positive and negative phototaxis of H. salinarum. Together, our results suggested a novel phototaxis signalling system in H. marismortui comprised of three sensory rhodopsins in which the phototactic response of SRI and SRII were attenuated by SRM.

A Unique Light-Driven Proton Transportation Signal in Halorhodopsin from Natronomonas pharaonis

Halorhodopsin (HR) is a seven-transmembrane retinylidene protein from haloarchaea that is commonly known to function as a light-driven inward chloride pump. However, previous studies have indicated that despite the general characteristics that most HRs share, HRs from distinct species differ in many aspects. We present indium-tin-oxide-based photocurrent measurements that reveal a light-induced signal generated by proton release that is observed solely in NpHR via purified protein-based assays, demonstrating that indeed HRs are not all identical. We conducted mutagenesis studies on several conserved residues that are considered critical for chloride stability among HRs. Intriguingly, the photocurrent signals were eliminated after specific point mutations. We propose an NpHR light-driven, cytoplasmic-side proton circulation model to explain the unique light-induced photocurrent recorded in NpHR. Notably, the photocurrent and various photocycle intermediates were recorded simultaneously. This approach provides a high-resolution method for further investigations of the proton-assisted chloride translocation mechanism.

Schiff Base Proton Acceptor Assists Photoisomerization of Retinal Chromophores in Bacteriorhodopsin

In this study, we investigated the ultrafast dynamics of bacteriorhodopsins (BRs) from Haloquadratum walsbyi (HwBR) and Haloarcula marismortui (HmBRI and HmBRII). First, the ultrafast dynamics were studied for three HwBR samples: wild-type, D93N mutation, and D104N mutation. The residues of the D93 and D104 mutants correspond to the control by the Schiff base proton acceptor and donor of the proton translocation subchannels. Measurements indicated that the negative charge from the Schiff base proton acceptor residue D93 interacts with the ultrafast and substantial change of the electrostatic potential associated with chromophore isomerization. By contrast, the Schiff base proton donor assists the restructuring of the chromophore cavity hydrogen-bond network during the thermalization of the vibrational hot state. Second, the ultrafast dynamics of the wild-types of HwBR, HmBRI, and HmBRII were compared. Measurements demonstrated that the hydrogen-bond network in the extracellular region in HwBR and HmBRII slows the photoisomerization of retinal chromophores, and the negatively charged helices on the cytoplasmic side of HwBR and HmBRII accelerate the thermalization of the vibrational hot state of retinal chromophores. The similarity of the correlation spectra of the wild-type HmBRI and D104N mutant of HwBR indicates that inactivation of the Schiff base proton donor induces a positive charge on the helices of the cytoplasmic side.