Chapter 11 Bacteriorhodopsin and related light-energy converters

The photochemical reaction cycle of retinal reconstituted bacteriorhodopsin

The function of three types of bacteriorhodopsins was compared: the wild-type, the bleached and retinal reconstituted and retinal deficient bacteriorhodopsin after retinal addition. The apparent pK(a) of the proton acceptor group for the bleached BR and retinal deficient BR shifted toward higher pH values compared to the wild-type BR. Fitting the photocycle model to the absorption kinetic signals for all three proteins showed the existence of the same intermediates, but the time-dependent concentration of the intermediates was different. Although measurements were made at pH 7, the absorption kinetics and photoelectric signals in both retinal reconstituted samples acted as wild-type bacteriorhodopsin at significantly higher pH. Below pH 3 the retinal deficient and reconstituted sample bleached. These results suggested that the added retinal was not able to rebind in the same position in the protein as in native bacteriorhodopsin. This points out that care should be taken, when bleached bacteriorhodopsin is reconstituted with different retinal analogs.

Photoelectric response of polarization sensitive bacteriorhodopsin films

Polarization sensitivity is introduced into oriented bacteriorhodopsin (BR) films through a photochemical bleaching process, which chemically modifies the structure of the purple membrane by breaking the intrinsic symmetry of the membrane-bound BR trimers. The resulting photovoltage generated in an indium-tin oxide (ITO)/BR/ITO detector is found to be anisotropic with respect to cross-polarized probe beams. A model, based on the polarization dependent photoselection of the BR molecules qualitatively explains the photochemical bleaching process and the observed anisotropic response. The effect reported here can be used to construct a polarization sensitive BR-based bio-photoreceiver.

Flash spectroscopic studies of the kinetics of the halorhodopsin photocycle

The photoreactions of halorhodopsin are complicated by the fact that the parent pigment and its photoproducts interact with chloride. Thus, in any photoreaction scheme at least four species have to be accounted for: HR565 and HR578 Cl-, as well as HR640 and HR520 Cl-. A photocycle scheme proposed earlier places the two main photointermediates of halorhodopsin, HR520 Cl- and HR640, into a single photocycle, with a chloride-dependent equilibrium between them [Oesterhelt, D., Hegemann, P., & Tittor, J. (1985) EMBO J. 4, 2351-2356]. This scheme, with the additional feature of direct photoproduction of HR640 from HR565, was tested in this work by using numerical solutions of the appropriate differential equations to simulate flash-induced absorption changes at 500 nm (production of HR520 Cl-) and at 660 nm (production of HR640). The time scale of the simulation was ms following the flash. Comparison of the simulated curves with experimental traces yielded a unique set of three rate constants. The proposed photocycle scheme and these rate constants predict well the shapes and amplitudes of flash traces at various chloride concentrations. It appears from the photocycle scheme, and the numerical values of rate constants, that chloride is bound with high affinity to the parent halorhodopsin molecule, but with much lower affinity to its main photointermediate. This may be the consequence of the fact that in the parent halorhodopsin in the retinal configuration is all-trans, but in the two photointermediates it is 13-cis.

An efficient system for the synthesis of bacteriorhodopsin in Halobacterium halobium

The mechanism by which bacteriorhodopsin (BR) transports protons across the cell membrane of Halobacterium halobium is actively studied in many laboratories. Currently available systems for the synthesis of mutant proteins obtained by site-directed mutagenesis of the gene encoding BR (bop) require reconstitution of the denatured polypeptide after its synthesis Escherichia coli or yeast; this approach is technically difficult and labor intensive, and raises questions about possible differences between in vivo and in vitro folding. Using a newly described transformation system and a halobacterial plasmid vector, we show that it is possible to reintroduce the bop gene into BR- strains of H. halobium. The bop-carrying plasmid expresses native BR in amounts similar to those obtained in several wild type strains. This system allows facile site-directed mutagenesis in halophilic archaebacteria.

The chromophore retinal hinders passive proton/hydroxide ion translocation through bacteriorhodopsin

Experiments have been performed to examine any influence of the chromophore retinal in bacteriorhodopsin (BR) on the passive proton/hydroxide ion flux through this integral membrane protein. BR was reconstituted into dimyristoylphosphatidylcholine (DMPC)-phosphatidylserine or DMPC-dimyristoylphosphatidylglycerol unilamellar vesicles with molar lipid to protein ratios ranging from 30 to 150. The entrapped fluorescence dye pyranine served as a reliable indicator of the internal proton concentration. Transmembrane pH-gradients were quickly established across the vesicular membrane and the kinetics of the induced fluorescence changes were compared for vesicles with incorporated native BR, BR bleached to the chromophore-free protein bacterioopsin, and BR regenerated from bacterioopsin with all-trans-retinal, respectively. For aggregated protein molecules, the H+/OH- diffusion across bacterioopsin was always considerably faster than that through the protein containing covalently bound retinal. The decay rate of the imposed pH-gradient was 4.4-9.1 and 2.0-5.1 times slower for native and regenerated BR, respectively, as compared to bacterioopsin. Stepwise regeneration of bacterioopsin with all-trans-retinal revealed a linear dependence of the predominant delta pH-decay time on the degree of regeneration. Essentially the same observations were made with monomeric protein molecules in vesicular lipid membranes. The results demonstrate that the chromophore retinal itself blocks the H+/OH- conducting pathway across the transmembrane protein BR or indirectly controls this path by inducing conformational changes in the protein upon binding.

Biosynthesis of the two halobacterial light sensors P480 and sensory rhodopsin and variation in gain of their signal transduction chains

The two retinal-containing photoreceptors of halobacteria, P480 and sensory rhodopsin, are formed constitutively and inducibly, respectively. Both photoreceptors are synthesized as apoproteins in cells with nicotine-inhibited retinal synthesis and are reconstituted as chromoproteins by the addition of all-trans retinal to cell membrane preparations. The decrease in photoreceptor-mediated photophobic response at the stationary growth phase of cells is not due to photoreceptor degradation but due to a deficiency of the signal transduction chain in the cell.

Topography of surface-exposed amino acids in the membrane protein bacteriorhodopsin determined by proteolysis and micro-sequencing

The topography of membrane-surface-exposed amino acids in the light-driven proton pump bacteriorhodopsin (BR) was studied. By limited proteolysis of purple membrane with papain or proteinase K, domains were cleaved, separated by SDS-PAGE, and electroblotted onto polyvinylidene difluoride (PVDF) membranes. Fragments transferred were sequenced in a gas-phase sequencer. Papain cleavage sites at Gly-65, Gly-72, and Gly-231, previously only deduced from the apparent molecular weight of the digestion fragments, could be confirmed by N-terminal micro-sequencing. By proteinase K, cleavage occurred at Gln-3, Phe-71, Gly-72, Tyr-131, Tyr-133, and Ser-226, i.e., in regions previously suggested to be surface-exposed. Additionally, proteinase-K cleavage sites at Thr-121 and Leu-127 were identified, which are sites predicted to be in the alpha-helical membrane-spanning segment D. Our results, especially that the amino acids Gly-122 to Tyr-133 are protruding into the aqueous environment, place new constraints on the amino-acid folding of BR across the purple membrane. The validity of theoretical prediction methods of the secondary structure and polypeptide folding for membrane proteins is challenged. The results on BR show that micro-sequencing of peptides separated by SDS-PAGE and blotted to PVDF can be successfully applied to the study of membrane proteins.

Bacteriorhodopsin precursor is processed in two steps

Besides the Coomassie-blue-stained band corresponding to mature bacterioopsin two additional bands of slightly higher apparent molecular masses were observed in purple membrane preparations from Halobacterium halobium by SDS-PAGE. The staining intensity within the triple band pattern varied with the age of the cell culture. For cells in the stationary growth phase the lower band, corresponding to mature bacterioopsin, is the predominant one. Immunodetection and site-specific proteolysis with papain identified the upper band as originating from the previously described precursor of bacterioopsin with its 13-amino-acid-long N-terminal presequence. Our results suggest that the intermediate band is due to a modified precursor of bacterioopsin with a truncated presequence of about eight amino acids. A two-step mechanism for the processing of pre-bacterioopsin to the mature protein in this archaebacterium is proposed.