Uniaxial Symmetry Breaking in Bacteriorhodopsin at the Thermal Phase Transition of Lipids of Purple Membranes

The article correlates between symmetry breaking and phase transition. An analogy, extending from physics to biology, is known to exist between these two topics. Bacteriorhodopsin (bR) as a paradigm of membrane proteins has been used as a case study in the present work. The bR, as the sole protein embedded in what is called a purple membrane (PM), has attracted widespread interest in bionanotechnological applications. The lipids of PM have a crucial role in maintaining the crystal lattice of bR inside PM. For this reason, the present work has been concerned with elucidating the thermal phase transition properties of the PM lipids in orthogonal directions. The results indicated that the axial symmetry of bR exhibits considerable changes occurring at the thermal phase transition of lipids. These changes are brought by an anomaly observed in the time course of orthogonal electric responses during the application of thermal fields on PM. The observed anomaly may bear on symmetry breaking in bR occurring at the phase transition of lipids based on such analogy found between symmetry breaking and phase transition. Lipid-protein interactions may underlie the broken axial symmetry of bR at such lipid thermal transition of PM. Accordingly, thermally perturbed axial symmetry of bR may be of biological relevance relying on the essence of the crystal lattice of bR. Most importantly, a question has to be raised in the present study: Can bR, as a helical protein with broken axial symmetry, affect the symmetry breaking of helical light? This may be of potential technical applications based on a recent discovery that bR breaks the symmetry of helical light.

Combined effect of the head groups and alkyl chains of archaea lipids when interacting with bacteriorhodopsin

Bacteriorhodopsin (bR), a transmembrane protein with seven α-helices, is highly expressed in the purple membrane (PM) of archaea such as Halobacterium salinarum. It is well known that bR forms two-dimensional crystals with acidic lipids such as phosphatidylglycerol phosphate methyl ester (PGP-Me)-a major component of PM lipids bearing unique chemical structures-methyl-branched alkyl chains, ether linkages, and divalent anionic head groups with two phosphodiester groups. Therefore, we aimed to determine which functional groups of PGP-Me are essential for the boundary lipids of bR and how these functionalities interact with bR. To this end, we compared various well-known phospholipids (PLs) that carry one of the structural features of PGP-Me, and evaluated the affinity of PLs to bR using the centerband-only analysis of rotor-unsynchronized spin echo (COARSE) method in solid-state NMR measurements and thermal shift assays. The results clearly showed that the branched methyl groups of alkyl chains and double negative charges in the head groups are important for PL interactions with bR. We then examined the effect of phospholipids on the monomer-trimer exchange of bR using circular dichroism (CD) spectra. The results indicated that the divalent negative charge in a head group stabilizes the trimer structure, while the branched methyl chains significantly enhance the PLs' affinity for bR, thus dispersing bR trimers in the PM even at high concentrations. Finally, we investigated the effects of PL on the proton-pumping activity of bR based on the decay rate constant of the M intermediate of a bR photocycle. The findings showed that bR activities decreased to 20% in 1,2-dimyristoyl-sn-glycero-3-phosphate (DMPA), and in 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) bilayers as compared to that in PM. Meanwhile, 1,2-Diphytanoyl-sn-glycero-3-phosphate (DPhPA) bilayers bearing both negative charges and branched methyl groups preserved over 80% of the activity. These results strongly suggest that the head groups and alkyl chains of phospholipids are essential for boundary lipids and greatly influence the biological function of bR.

Comparison of functionality and structural stability of bacteriorhodopsin reconstituted in partially fluorinated dimyristoylphosphatidylcholine liposomes with different perfluoroalkyl chain lengths

Amphiphilic molecules with one or more perfluoroalkyl groups (Rf, C_nF_2n+1), which show peculiar interfacial properties, are attracting much attention in membrane protein science. We recently have developed a partially fluorinated dimyristoylphosphatidylcholine (DMPC) with a perfluorobutyl group in the hydrophobic chain terminal (F4-DMPC) and demonstrated that F4-DMPC is a promising material for incorporating membrane proteins. Moreover, we have found out that membrane properties of a series of partially fluorinated DMPCs with different Rf chain lengths (Fn-DMPCs) vary in a significant Rf chain length-dependent manner. In the present study, structural and functional properties of a membrane protein bacteriorhodopsin (bR) in the Fn-DMPC (n = 4, 6, and 8) membranes (bR/Fn-DMPC) are investigated using several physicochemical techniques. Regardless of the Rf chain lengths, bR/Fn-DMPCs retain native-like structural and functional properties at 30 °C, unlike bR molecules in DMPC vesicles. In particular, bR/F6-DMPC, which is in the fluid phase at 30 °C, shows flash-induced transient absorption changes very similar to the native purple membrane (PM) and very high thermal stability of bR trimers comparable to the PM. Structural and functional properties of bR/Fn-DMPCs are discussed compared to the PM and bR/DMPC.

Archaeal Glycolipid S-TGA-1 Is Crucial for Trimer Formation and Photocycle Activity of Bacteriorhodopsin

Although it has been demonstrated that membrane proteins (MPs) require lipids to ensure their structural and functional integrity, details on how lipid-MP interactions regulate MPs are still unclear. Recently, we developed a concise method for quantitatively evaluating lipid-MP interactions and applied it to bacteriorhodopsin (bR), a halobacterial MP that forms trimers and acts as a light-driven proton pump. Consequently, we found that the halobacterial glycolipid, S-TGA-1, has the highest affinity for bR, among other lipids. In this study, we examined the effects of S-TGA-1 on bR via visible circular dichroism spectroscopy, flash photolysis, and proton influx measurement. The results showed that S-TGA-1 efficiently promotes trimer formation, photocycle, and proton pumping in bR. Our data also suggested that the bR photocycle is restored as a consequence of the trimerization induced by the lipid. This study demonstrates clearly that lipids specifically interacting with MPs can have significant impacts on MP structure and/or function. The methodology adopted in our studies can be applied to other MPs and will help elucidate the physiological functions of lipids in terms of lipid-MP interactions, thus accelerating "lipid chemical biology" studies.

Lateral Diffusion and Molecular Interaction in a Bilayer Membrane Consisting of Partially Fluorinated Phospholipids

Fluorinated lipids and surfactants are attractive biomimetic materials for the extraction and reorganization of membrane proteins because of the biological inertness of fluorocarbons. We investigated the fundamental physical properties of a partially fluorinated phospholipid (F4-DMPC), such as phase transition, area thermal expansion, and lateral lipid diffusion, to evaluate the intermolecular interaction of F4-DMPC in the hydrophobic region quantitatively on the basis of free-volume theory. Fluorescence microscope observation of the supported lipid bilayer (SLB) of F4-DMPC showed that the phase transition between the liquid crystalline and gel phases occurred at 5 °C and that the area thermal expansion coefficient was independent of the temperature near the phase transition temperature. We performed a single particle tracking of the F4-DMPC-SLB on a SiO₂/Si substrate, to measure the diffusion coefficient and its temperature dependence. The apparent activation energy (E'_a) of lateral lipid diffusion, which is an indicator of intermolecular interaction, was 39.1 kJ/mol for F4-DMPC, and 48.2 kJ/mol for a nonfluorinated 1,2-dioleoyl-sn-glycero-3-phosphocholine as a control. The difference of 9 kJ/mol in E'_a was significant compared with the difference due to the acyl chain species among nonfluorinated phosphatidylcholine and also that caused by the addition of cholesterol and alcohol in the bilayer membranes. We quantitatively evaluated the attenuation of intermolecular interaction, which results from the competition between the dipole-induced packing effect and steric effect at the fluorocarbon segment in F4-DMPC.

Stereoselective synthesis of the head group of archaeal phospholipid PGP-Me to investigate bacteriorhodopsin–lipid interactions

Phosphatidylglycerophosphate methyl ester (PGP-Me), a major constituent of the archaeal purple membrane, is essential for the proper proton-pump activity of bacteriorhodopsin (bR).