The photoreceptor sensory rhodopsin I as a two-photon-driven proton pump.

Abstract

Proton translocation experiments with intact cells of Halobacterium salinarium overproducing sensory rhodopsin I (SRI) revealed transport activity of SRI in a two-photon process. The SRI amino acid sequence (9) reveals a striking degree of sequence homology to BR (10) and HR (11), suggesting seven transmembrane helices (A-G) as found for BR (12) and HR (13). Retinal is bound as a protonated Schiff base (14,15) to Lys-206 in helix G of SRI. In BR Asp-85 and Asp-96 function as proton acceptor and donor, respectively (16-18).The absence of one or both of these residues renders the proton pump inefficient for phototrophic growth, because turnover rates drop from about 100 s-1 to 1 s-1 in the case of the Asp-96 -> Asn mutation, whereas in Asp-85 -* Xaa mutants, M formation (deprotonation of the Schiff base) is prevented. In SRI the corresponding positions are occupied byThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.Asp-76 and Tyr-87; i.e., the amino acid functioning as acceptor in BR is retained while the donor site is replaced by tyrosine, thus explaining the low turnover rate of the SRI photocycle. In addition, Fourier transform IR spectroscopy studies (19,20) have shown that Asp-76 is protonated in the initial state as well as in SRI380, undergoing only a change in environment during the transition. A possible ion translocation activity of SRI could not be excluded on this basis, because BR mutants lacking both donor and acceptor side chain are still active at a low level of transport rates in a two-photon process (21). However, a previous study of SRI photocycling (22) did not detect any electrogenic activity of SRI.Until recently, experiments on SRI were carried out with wild-type cells and derived membrane fractions where SRI occurs as a stoichiometric complex with its transducer protein . The thermal decay of SRI380 is independent of bulk pH in the case of the HtrI-SRI complex, while an exponential increase in the decay time of SRI380 with increasing pH is observed for SRI alone. In the case of Htr-deficient SRI, the formation of SRI380 can be correlated with the appearance of one proton in the bulk medium per SRI380 formed (26), but at the same time the recovery of the initial state is slowed to about 30 s at pH 7 (27). Thus, a one-photon process could only drive proton translocation at very slow speed but a two-photon process might be efficient enough to be detected in intact cells by standard techniques.Here we report the results of experiments with cells overproducing HtrI-free SRI by a factor of 50 (28). SRI is indeed shown to be active in proton translocation in a two-photondriven process involving the two photocycle intermediates SRI380 and SRI520.MATERIALS AND METHODS Bacterial Strains. H. salinarium strain Q6, used for transport experiments and cell membrane preparations, is a SRIoverproducing strain which has been constructed by tra...

Behavioural responses of bacteria to light and oxygen

“…In the absence of HtrI, the SRI protein shows slow proton pumping via the retinal Schiff's base (Bogomolni et al 1994). This result suggests that the photochemical cycle undergone by the SR proteins may normally result in a Schiff's base deprotonation-reprotonation reaction similar to that shown by bacteriorhodopsin (BR), but that the presence of HtrI prevents the release of protons to the aqueous phase (Haupts et al 1995). Instead, HtrI presumably undergoes a conformational change by either electrostatic or allosteric coupling and this results in a photoresponse.…”

Five Residues in the HtrI Transducer Membrane-proximal Domain Close the Cytoplasmic Proton-conducting Channel of Sensory Rhodopsin I

“…In the blue species ( max 587 nm) Asp-76 is protonated, and in the purple species ( max 552) Asp-76 is unprotonated. In HtrI-free SRI, the pK a of Asp-76 is 7.2-7.4 (6,8). Binding of full-length HtrI, in addition to blocking the SRI cytoplasmic channel, increases this pK a of Asp-76 to 8.6 -8.9 (25).…”

Electrophysiological characterization of specific interactions between bacterial sensory rhodopsins and their transducers

“…The capability of SRI to pump protons has been studied in greater detail (5)(6)(7)(8). Olson et al (5) and Bogomolni et al (6) demonstrated that the proton transfer is driven by orange light in a single photon process.…”

“…Haupts et al (7,8) performed similar experiments with intact cells and, additionally, analyzed photocurrents of SRI containing membranes attached to the black-lipid membrane. From their results, the authors concluded that the proton transfer is based on a two-photon process in which the S 373 -intermediate absorbs a ''blue'' photon to short-cut the photocycle.…”