Chemical modification of bacteriorhodopsin by fluorescamine

Asymmetric distribution of biotin labeling on the purple membrane

This work examined the biotin modification of bacteriorhodopsin (BR) in the purple membrane (PM). The results of flash kinetic absorption measurements showed that photocycle was maintained in biotinylated BR. Biotinylated BR also maintained its photoelectric activity, as indicated by the photoelectric response of the bilayer lipid membrane (BLM). Atomic force microscopy (AFM) of stretavidiin-bound biotin revealed that biotin molecules covered both surfaces of the, but the amount of biotinylated BR on the extracellular (EC) surface was markedly higher than on the cytoplasmic (CP) surface. Further studies showed that, after reaction with fluorescamine (FL), biotin labeling occurred only on the CP surface. These results are informative for future work on bioconjugation of BR as well as work on oriented assembly and the design of BR-based photoelectric devices.

pH dependence of the absorption spectra and photochemical transformations of the archaerhodopsins

Two strains of archaebacteria have been found to contain light-driven proton pumping pigments analogous to bacteriorhodopsin (bR) in Halobacterium salinarium. These proteins are called archaerhodopsin-1 (aR-1) and archaerhodopsin-2 (aR-2). Their high degree of sequence identity with bR within the putative proton channel enables us to draw some conclusions about the roles of regions where differences in amino acids exist, and in particular the surface residues, on the structure and function of retinal-based proton pumps. We have characterized the spectral and photochemical properties of these two proteins and compared them to the corresponding properties of bR. While there are some differences in absorbance maxima and kinetics of the photocycle, most of the properties of aR-1 and aR-2 are similar to those of bR. The most striking differences of these proteins with bR are the lack of an alkaline-induced red-shifted absorption species and a dramatic (apparent) decrease in the light-induced transient proton release. In membrane sheet suspensions of aR-1 at 0.15 M KCl, the order of proton release and uptake appears opposite that of bR, in which proton release precedes uptake. The nature of this behavior appears to be due to differences in the amino acid sequence at the surfaces of the proteins. In particular, the residue corresponding to the lysine at position 129 of the extracellular loop region of bR is a histidine in aR-1 and could regulate the efficient release of protons into solution in bR.

Location of lysine-129 and lysine-40/41 with respect to retinylidene chromophore in bacteriorhodopsin

Fluorescence properties of fluorescamine-modified bacteriorhodopsin (BR) have been studied. BR reacts with fluorescamine (1:2 ratio) to give protein (FLBR-I) modified at Lys-129. By making use of citraconic anhydride as a masking reagent, fluorescamine modification of Lys-40/41 in BR has been achieved. Forster's resonance energy transfer studies indicate that the distance between FL-Lys-129 and retinylidene chromophore is 11 A, whereas that between FL-Lys-40/41 and retinylidene chromophore is 24 A. These measured distances have been analysed in terms of BR structure.