Methods for mutagenesis of the bacterioopsin gene

Bacteriorhodopsin oriented in polyvinyl alcohol films as an erasable optical storage medium

Films of oriented bacteriorhodopsin have been formed in polyvinyl alcohol with excellent optical quality. Images with high contrast have been impressed and erased on these films. Second-harmonic microscopy has been used to read the image on a bacteriorhodopsin-polyvinyl alcohol film without erasure. The potential of these films for molecular information storage and computation is discussed.

Effects of individual genetic substitutions of arginine residues on the deprotonation and reprotonation kinetics of the Schiff base during the bacteriorhodopsin photocycle

The rates are determined for the deprotonation and reprotonation of the protonated Schiff base (PSB) as well as of formation and decay of the UV transient in the photocycle of seven bacteriorhodopsin (bR) mutants in which Arg-7, 82, 164, 175, 225, or 227 are replaced by glutamine and Arg-134 by cysteine. The results show that all these mutations increase the rate of deprotonation of the PSB compared to ebR, (wild-type bacteriorhodopsin expressed in Escherichia coli) greatly increase the rate of the reprotonation of the SB (Schiff base) in the case of the Arg-164 and Arg-175 mutations and dramatically decrease this rate in the case of the Arg-227 mutation. Temperature studies on the latter mutant suggest that the observed change in its rate of reprotonation is due to large decrease in the energy and entropy of activation, similar to those observed for Asp-96 mutations (Miller, A. and D. Orsterhelt. 1990. Biochim. Biophys. Acta. 1020:57-64). These results suggest that the reprotonation process is changed to a proton diffusion-controlled mechanism in the Arg-227 mutant due to a change in the structure of the proton channel. The absorption intensity ratio (AUV/AMslow) of each arginine mutant relative to that of ebR is found to be similar to that for native purple membrane (PM) except for the Arg-227 mutant where it is greatly reduced, and for the Arg-82 mutant where it is not observed, suggesting that both Arg-227 and Arg-82 residues somehow play roles in inducing the UV transient absorption. All the above results are discussed in terms of the model for the structure of bR proposed by Henderson, R., J.M. Baldwin, T.A. Ceska, F. Zemlin, E. Beckmann, and K.H. Downing. (1990. J. Mol. Biol. 213:899-929).

Effect of genetic modification of tyrosine-185 on the proton pump and the blue-to-purple transition in bacteriorhodopsin

The retinylidene chromophore mutant (Y185F) of bacteriorhodopsin, in which Tyr-185 is substituted by phenylalanine, is examined and compared with wild-type bacteriorhodopsin expressed in Escherichia coli; both were reinstituted similarly in vesicles. The Y185F mutant shows (at least) two distinct spectra at neutral pH. Upon light absorption, the blue species (which absorbs in the red) behaves as if "dead"--i.e., neither its tyrosine nor its protonated Schiff base undergoes deprotonation nor does its tryptophan fluorescence undergo quenching. This result is unlike either the purple species (which absorbs in the blue) or wild-type bacteriorhodopsin expressed in E. coli. As the pH increases, both the color changes and the protonated Schiff base deprotonation efficiency suggest a blue-to-purple transition of the Y185F mutant near pH 9. If this blue-to-purple transition of Y185F corresponds to the blue-to-purple transition of purple-membrane (native) bacteriorhodopsin (occurring at pH 2.6) and of wild-type bacteriorhodopsin expressed in E. coli (occurring at pH 5), the protein-conformation changes of this transition as well as the protonated Schiff base deprotonation may be controlled not by surface pH alone, but rather by the coupling between surface potential and the general protein internal structure around the active site. The results also suggest that Tyr-185 does not deprotonate during the photocycle in purple-membrane bacteriorhodopsin.

Vibrational spectroscopy of bacteriorhodopsin mutants: I. Tyrosine-185 protonates and deprotonates during the photocycle

The techniques of FTIR difference spectroscopy and site-directed mutagenesis have been combined to investigate the role of individual tyrosine side chains in the proton-pumping mechanism of bacteriorhodopsin (bR). For each of the 11 possible bR mutants containing a single Tyr----Phe substitution, difference spectra have been obtained for the bR----K and bR----M photoreactions. Only the Tyr-185----Phe mutation results in the disappearance of a set of bands that were previously shown to be due to the protonation of a tyrosinate during the bR----K photoreaction [Rothschild et al.: Proceedings of the National Academy of Sciences of the United States of America 83:347, (1986]). The Tyr-185----Phe mutation also eliminates a set of bands in the bR----M difference spectrum associated with deprotonation of a Tyr; most of these bands (e.g., positive 1272-cm-1 peak) are completely unaffected by the other ten Tyr----Phe mutations. Thus, tyrosinate-185 gains a proton during the bR----K reaction and loses it again when M is formed. Our FTIR spectra also provide evidence that Tyr-185 interacts with the protonated Schiff base linkage of the retinal chromophore, since the negative C = NH+ stretch band shifts from 1640 cm-1 in the wild type to 1636 cm-1 in the Tyr-185----Phe mutant. A model that is consistent with these results is that Tyr-185 is normally ionized and serves as a counter-ion to the protonated Schiff base. The primary photoisomerization of the chromophore translocates the Schiff base away from Tyr-185, which raises the pKa of the latter group and results in its protonation.