Transmembrane orientation of α-helices in the thylakoid membrane and in the light-harvesting complex. A polarized infrared spectroscopy study

Pigment-pigment interactions and secondary structure of reconstituted algal chlorophyll a/b-binding light-harvesting complexes of Chlorella fusca with different pigment compositions and pigment-protein stoichiometries

Earlier we have shown by in vitro reconstitution experiments that the pigment composition of the chlorophyll alb-binding light-harvesting complex of the green alga Chlorella fusca could be altered in a relatively broad range (Meyer and Wilhelm 1993). In this study we used these reconstituted complexes of different pigment loading to analyze the excitonic interactions between the pigment molecules and the secondary structure by means of circular dichroism spectra in the visible and the far UV spectral regions, respectively. We found that, in contrast to the expectations, the pigment composition and pigment content hardly affected the circular dichroism spectra in the visible spectral region. Reconstituted complexes, independent of their pigment composition, exhibited the most characteristic circular dichroism bands of the native light-harvesting complex, even if one polypeptide bound only 3 chlorophyll a, 3 chlorophyll b and 1-2 xanthophyll molecules. Full restoration of the protein secondary structure, however, could not be achieved. The α-helix content depended significantly on the pigment composition as well as on the pigment-protein ratio of the reconstituted complexes. Further binding of pigments resulted in restoration of the minor excitonic circular dichroism bands, the amplitudes of which depended on the pigment content of the reconstituted complexes. These data suggest that in the reconstitution of light-harvesting complexes a 'central cluster' of pigment molecules plays an important role. Further binding of pigments to the 'peripheral binding sites' appeared also to stabilize the protein secondary structure of the reconstituted complexes.

Structure and orientation of apo B-100 peptides into a lipid bilayer

Peptides corresponding to lipid binding domains of Apo B-100 were synthesized, purified, and incubated with dimyristoylphosphatidylcholine (DMPC) liposomes. The secondary structure of the apo B-100 peptide-lipid complexes was evaluated by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). Those peptides belonging to the hydrophobic "core" domain of apo B-100 when associated with phospholipids were rich in beta sheet structure; a predominant alpha helical conformation was shown to be associated with one peptide located in a surface region of apo B-100. IR dichroic spectra revealed, in the case of the "core" peptides, that the beta sheet component is the only oriented structure with respect to the phospholipid acyl chains. This orientation of the beta sheet was recently found in LDL particles after proteolytic digestion by trypsin (Goormaghtigh, E., Cabiaux, V., De Meutter, J., Rosseneu, M., and Ruysschaert, J. M., 1993, Biochemistry 32, 6104-6110). Altogether, the data suggest that beta sheet, present in a high proportion in the native apo B-100, is probably another protein structure in addition to the amphipathic helix which strongly interacts with the lipid outer layer surrounding the LDL particle.

Pigments induce folding of light-harvesting chlorophyll a/b-binding protein

The conformational behaviour of the light-harvesting chlorophyll a/b-binding protein (LHCP), the apoprotein of the major light-harvesting complex (LHCII) of photosystem II in plants, has been studied. According to the circular dichroism in the ultraviolet range measured with isolated LHCII, the protein in the complex adopts a folded structure with a high content of alpha helix (about 60%), whereas the non-pigmented, solubilized protein has a less ordered structure (about 20% alpha helix). LHCP-pigment complexes that have been reconstituted from the overexpressed protein and isolated pigments in the presence of detergents display a protein CD signal similar to that of authentic LHCII, indicating that LHCP folds into the native structure during the reconstitution procedure. Renaturation of LHCP in these experiments is dependent on the presence of pigments and the formation of stable LHCP-pigment complexes. Pigment-induced engagement of LHCP in a compact structure has also been shown by two additional experimental approaches. (a) Upon complex formation, LHCP or its precursor (pLHCP) becomes resistant to trypsin digestion with the exception of an N-terminal segment of the protein; the same protection of LHCP is known to occur in intact thylakoids. (b) Pigment binding renders a cysteine residue within the N-proximal hydrophobic domain of the protein as well as a newly introduced cysteine four amino acid positions from the C terminus inaccessible to modification with a sulfhydryl-specific label whereas the N terminus stays susceptible to specific labelling. These observations support the notion that only the N terminus protrudes from a compact protein-pigment structure in LHCII. The fact that the major part of LHCP is trypsin-resistant in pigmented complexes reconstituted in the absence of a membrane or even lipids justifies caution in using protection against trypsin as a criterion for the integration of LHCP into the thylakoid membrane.

Structure and orientation of the surfactant-associated protein C in a lipid bilayer

The secondary structure of native and depalmitoylated porcine surfactant-associated protein C (SP-C) was studied by attenuated total reflection Fourier-transform infrared spectroscopy. Both forms of porcine SP-C adopt mainly an alpha-helical conformation. These two forms of the protein were reconstituted in a lipid bilayer. The insertion of the protein in a membrane is associated with an increase of the alpha-helical content. Dichroic measurements show that, in both cases, the long axis of the alpha-helix is oriented parallel to the lipid acyl chains.

Protein and chlorophyll in photosystem II probed by infrared spectroscopy

The infrared spectra of photosystem II (PS II) enriched submembrane fractions isolated from spinach are obtained in water and in heavy water suspension Other spectra are obtained after a photooxidation reaction was performed on PS II to bleach the pigments. The water bands are removed by computer subtraction and the amide bands (A, B, I, II, and III) of the protein are identified. Computer enhancement techniques are used to narrow the bandwidth of the bands that the weak chlorophyll bands, buried in the much stronger protein bands, can be observed. Comparing the spectra of native and photooxidized PS II pr in water and in heavy water, we determine that three polypeptide domains are present in the native material. The first domain, which contains 22% of th is situated in the peripheral region of the PS II system. The polypeptides in this region are unfolded and devoid of chlorophyll. The second domain con of the polypeptides, is more organized, and contains the chlorophylls. The third domain has an alpha-helix configuration, does not contain chlorophyll, a affected by the photooxidation reaction or by the proton/deuteron exchange. Three different types of chlorophyll organisation are identified: two have carbonyl groups non-bonded, differing from one another only in their hydrophobic milieux; the third is weakly bonded to another unidentified group. Other forms of chlorophyll organisation are present but could not be observed because their absorption is buried in the protein amide I band.

Two-dimensional structure of plant light-harvesting complex at 3.7 A [corrected] resolution by electron crystallography

The structure of the light-harvesting chlorophyll a/b-protein complex has been determined at 3.7 A resolution in projection by electron diffraction, electron microscopy and image analysis. Diffraction patterns and high-resolution spotscan images of two-dimensional crystals stabilized with tannin were recorded at low temperature. Phases of structure factors were obtained directly by image processing, after correction of the images for lattice distortions, defocus and beam tilt. Amplitudes were measured by electron diffraction. The projection map shows the detailed structure of the trimeric complex, suggesting the positions of two domains of potential structural and functional homology, of one membrane-spanning alpha-helix approximately perpendicular to the membrane plane and of several tightly bound lipid molecules.

Structure of light-harvesting chlorophyll a/b protein complex from plant photosynthetic membranes at 7 A resolution in projection

The structure of thin three-dimensional crystals of the light-harvesting chlorophyll a/b protein complex, an integral membrane protein from the photosynthetic membrane of chloroplasts, has been determined at 7 A (1 A = 0.1 nm) resolution in projection. The structure analysis was carried out by image processing of low-dose electron micrographs, and electron diffraction of thin three-dimensional crystals preserved in tannin. The three-dimensional crystals appeared to be stacks of two-dimensional crystals having p321 symmetry. Results of the image analysis indicated that the crystals were disordered, due to random translational displacement of stacked layers. This was established by a translation search routine that used the low-resolution projection of a single layer as a reference. The reference map was derived from the symmetrized average of two images that showed features consistent with the projected structure of negatively stained two-dimensional crystals. The phase shift resulting from the displacement of each layer was corrected. Phase shifts were then refined by minimizing the phase residual, bringing all layers to the same phase origin. Refined phases from different images were in agreement and reliable to 7 A resolution. A projection map was generated from the averaged phases and electron diffraction amplitudes. The map showed that the complex was a trimer composed of three protein monomers related by 3-fold symmetry. The projected density within the protein monomer suggested membrane-spanning alpha-helices roughly perpendicular to the crystal plane. The density in the centre and on the periphery of the trimeric complex was lower than that of the protein, indicating that this region contained low-density matter, such as lipids and antenna chlorophylls.

Orientational properties of biological pigments in ordered systems studied with polarized light: photosynthetic pigment-protein complexes in membranes

A discussion is presented of the problems involved in the interpretation of linear dichroism and fluorescence depolarization experiments on macroscopically ordered membrane systems. Particular attention has been paid to ordered membranes containing photosynthetic pigment-protein complexes, but the mathematical treatment can equally well be applied to other systems. The information about the orientational properties of the pigments is obtained by the application of the theories developed for the characterization of the molecular orientational order in liquid-crystalline materials. It is shown that while linear dichroism only yields the order parameter S mu of the absorption transition moment, fluorescence depolarization experiments yield in addition the order parameter Sv of the emission transition moment as well as three orientational correlation functions of the two transition moments. It is argued that in general the latter information can only be obtained on utilizing a number of experimental scattering geometries. In particular, the merits of angle-resolved experiments are illustrated.

Three-dimensional crystals of the light-harvesting chlorophyll a/b protein complex from pea chloroplasts

Two forms of three-dimensional crystals of the light-harvesting chlorophyll a/b protein complex from pea have been obtained. Crystals of one form grew as hexagonal plates measuring up to 150 micron across and 2 to 3 micron in thickness. Electron diffraction patterns of thin hexagonal plates showed sharp reflections to a resolution of 3.7 A on a hexagonal reciprocal lattice. The unit cell in projection (a = 127.0 A) and the symmetry of the diffraction pattern (6 mm) suggested that the hexagonal plates were highly ordered stacks of two-dimensional crystals suitable for structure analysis by electron microscopy and image processing. Crystals of a second form grew as dark green octahedra measuring roughly 0.5 mm across. Low-resolution X-ray diffraction patterns suggested a large cubic unit cell (a = 390 A). SDS/polyacrylamide gel electrophoresis of single octahedral crystals showed the same polypeptide composition as the starting solution, one major band at 24,000 apparent molecular weight and two satellite bands of 23,000 and 23,500 apparent molecular weight.

Orientation of gramicidin D incorporated into phospholipid multibilayers: a Fourier transform infrared-attenuated total reflection spectroscopic study

Polarized Fourier transform infrared (FTIR)-attenuated total reflection (ATR) spectroscopy was applied to study the orientation of the linear pentadecapeptide antibiotic gramicidin D incorporated into phospholipid multibilayers, which were cast on a germanium ATR plate from chloroform solution. In DMPC and DPPC multibilayers, the CH2 stretching bands of lipid hydrocarbon chains were slightly shifted to the higher frequency side and bandwidth was increased in the presence of gramicidin. However, in DPPE multibilayers, frequencies and bandwidths of these bands were unaltered. In each case, gramicidin produced little effect on the orientation of lipid hydrocarbon chains, suggesting that gramicidin penetrates into lipid layers without noticeable perturbations. Upon incubation of cast films in contact with water above the gel-liquid-crystalline transition temperature (Tc) of lipids, the reorientation of gramicidin in lipid multibilayers occurred, the degree thereof depending upon the fluidity of the lipid hydrocarbon chains and the amount of surrounding water. In DMPC multibilayers, the helix axis of gramicidin was oriented almost parallel to the lipid hydrocarbon chains after incubation. In DPPC multibilayers, on the other hand, the helix axis of gramicidin was tilted on average about 15 degrees from the lipid hydrocarbon chains after incubation. However, in DPPE multibilayers, which are known to have the most rigid bilayer structures, the reorientation of gramicidin could not be seen.

Polarized photoacoustic, absorption and fluorescence spectra of chloroplasts and thylakoids oriented in polyvinyl alcohol films

The polarized photoacoustic, absorption and fluorescence spectra of chloroplasts and thylakoids in unstretched and stretched polyvinyl alcohol films were measured. The intensity ratios of fluorescence bands at 674 nm, 700 nm, 730 nm and 750 nm, and the polarized fluorescence excitation spectra are strongly dependent on light polarization and film stretching. In stretched films, thylakoids exhibit predominantly 674 nm emission. The ratio of photoacoustic signal to absorption is different for light polarized parallel and perpendicular to film stretching. This difference is large in the region of chlorophyll a and carotenoids absorption in which the fluorescence excitation spectra are also strongly dependent on light polarization and film stretching. The observed spectral changes are explained by reorientation of pigment molecules influencing the yield of excitation transfer between different pigments.