Role of Salt in Reconstituting Photocycle Behavior in Triton-Damaged Purple Membranes by Addition of Native Lipids

Interconversions among four M-intermediates in the bacteriorhodopsin photocycle

Halobacterium salinarum displays four distinct kinetic forms of M-intermediate in its bacteriorhodopsin photocycle. In wild-type, there are mainly two species with time constants near 2 and 5 ms. Under various kinds of stress, two other species arise with time constants near 10 and 70 ms. We show that these four species are interconvertible. Increases in membrane hydrophobicity convert the slower to faster forms. Perturbations caused by Triton X-100 or mutations convert faster to slower forms. The fastest form requires a hydrophobic membrane environment near a ring of four charged aspartate residues in the trimer, namely Asp36, Asp38, Asp102, and Asp104 in the cytoplasmic loop regions. Interconversions of the 2-ms and 5-ms species of the wild-type are accomplished by pH-changes. The potential significance of these findings is discussed.

Purple membrane lipid control of bacteriorhodopsin conformational flexibility and photocycle activity

Specific lipids of the purple membrane of Halobacteria are required for normal bacteriorhodopsin structure, function, and photocycle kinetics [Hendler, R.W. & Dracheva, S. (2001) Biochemistry (Moscow)66, 1623-1627]. The decay of the M-fast intermediate through a path including the O intermediate requires the presence of a hydrophobic environment near four charged aspartic acid residues within the cytoplasmic loop region of the protein (R. W. Hendler & S. Bose, unpublished results). On the basis of the unique ability of squalene, the most hydrophobic purple membrane lipid, to induce recovery of M-fast activity in Triton-treated purple membrane, we proposed that this uncharged lipid modulates an electrostatic repulsion between the membrane surface of the inner trimer space and the nearby charged aspartic acids of the cytoplasmic loop region to promote transmembrane alpha-helical mobility with a concomitant increase in the speed of the photocycle. We examined Triton-treated purple membranes in various stages of reconstitution with native lipid suspensions using infrared spectroscopic techniques. We demonstrate a correlation between the vibrational half-width parameter of the protein alpha-helical amide I mode at 1660 cm-1, reflecting the motional characteristics of the transmembrane helices, and the lipid-induced recovery of native bacteriorhodopsin properties in terms of the visible absorbance maxima of ground state bacteriorhodopsin and the mean decay times of the photocycle M-state intermediates.

The laser-induced blue state of bacteriorhodopsin: mechanistic and color regulatory roles of protein-protein interactions, protein-lipid interactions, and metal ions

In this paper we characterize the mechanistic roles of the crystalline purple membrane (PM) lattice, the earliest bacteriorhodopsin (BR) photocycle intermediates, and divalent cations in the conversion of PM to laser-induced blue membrane (LIBM; lambda(max)= 605 nm) upon irradiation with intense 532 nm pulses by contrasting the photoconversion of PM with that of monomeric BR solubilized in reduced Triton X-100 detergent. Monomeric BR forms a previously unreported colorless monomer photoproduct which lacks a chromophore band in the visible region but manifests a new band centered near 360 nm similar to the 360 nm band in LIBM. The 360 nm band in both LIBM and colorless monomer originates from a Schiff base-reduced retinyl chromophore which remains covalently linked to bacterioopsin. Both the PM-->LIBM and monomer-->colorless monomer photoconversions are mediated by similar biphotonic mechanisms, indicating that the photochemistry is localized within single BR monomers and is not influenced by BR-BR interactions. The excessively large two-photon absorptivities (> or =10(6) cm(4) s molecule(-1) photon(-1)) of these photoconversions, the temporal and spectral characteristics of pulses which generate LIBM in high yield, and an action spectrum for the PM-->LIBM photoconversion all indicate that the PM-->LIBM and Mon-->CMon photoconversions are both mediated by a sequential biphotonic mechanism in which is the intermediate which absorbs the second photon. The purple-->blue color change results from subsequent conformational perturbations of the PM lattice which induce the removal of Ca(2+) and Mg(2+) ions from the PM surface.

Regulation of the bacteriorhodopsin photocycle and proton pumping in whole cells of Halobacterium salinarium

Single-turnover kinetics of the bacteriorhodopsin photocycle and proton-pumping capabilities of whole cells were studied. It was found that the Delta mu (tilde)H+ of the cell had a profound influence on the kinetics and components of the cycle. For example, comparing the photocycle in whole cells to that seen in PM preparations, we found that (1) the single-turnover time of the cycle was increased approximately 10-fold, (2) the mole fraction of M-fast (at high actinic light) decreased from 50 to 20%, and (3) the time constant for M-slow increased significantly. The level of Delta mu(tilde)H+ was dependent on respiration, ATP formation and breakdown, and the magnitude of a pre-existing K+ diffusion gradient. The size of the Delta mu(tilde)H+ could be manipulated by additions of HCN, nigericin, and DCCD (N,N'-dicyclohexylcarbodamide). At higher levels of Delta mu(tilde)H+, further changes in the photocycle were seen. (4) Two slower components of M-decay appeared as major components. (5) The apparent conversion of the M-fast to the O intermediate disappeared. (6) A partial reversal of an early photocycle step occurred. The photocycle of intact cells could be changed to that seen in purple membrane suspensions by the energy-uncoupler CCCP or by lysis of the cells. In fresh whole cells, light-induced proton pumping was not seen until the K+ diffusion potential was dissipated and proton accumulation facilitated by use of a K+-H+ exchanger (nigericin), respiration was inhibited by HCN, and ATP synthesis and breakdown were inhibited by DCCD. In stored cells, the pre-existing K+ diffusion gradient was diminished through slow diffusion, and only DCCD and HCN were required to elicit proton extrusion.

Importance of specific native lipids in controlling the photocycle of bacteriorhodopsin

Brief treatment of purple membrane (PM) with dilute detergent can cause major disruption of the BR photocycle without disrupting the trimer structure of BR [Mukhopadhyay et al. (1996) Biochemistry 35, 9245-9252]. Normal photocyle behavior can be recovered by incubating the damaged membranes with a total extract of the five types of native lipids present in PM. It is shown here that full restoration can also be obtained with combinations of squalene (SQ) and phosphatidyl glycerophosphate (PGP) which act synergistically. The addition of SQ to suboptimal levels of PGP induces complete reconstitution, principally by restoring the characteristics of the fast M intermediate, Mf (as defined in Mukhopadhyay et al. (1996) Biochemistry 35, 9245-9252). The addition of small amounts of PGP to SQ, which alone is ineffective, also induces full reconstituion. At very high levels, full reconstitution can be obtained with PGP alone. These results, in combination with earlier studies which implicate an acidic amino acid residue [Bose et al. (1997) J. Phys. Chem. B 101, 10584-10587], suggest that a crucial interaction between a particular amino acid residue and a SQ-PGP lipid complex may be essential for normal BR photocycle activity.