The light-harvesting complex of photosystem II in barley. Structure and chlorophyll organization

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.

Pigment and protein composition of reconstituted light-harvesting complexes and effects of some protein modifications

The structure and heterogeneity of LHC II were studied by in vitro reconstitution of apoproteins with pigments (Plumley and Schmidt 1987, Proc Natl Acad Sci 84: 146-150). Reconstituted CP 2 complexes purified by LDS-PAGE were subsequently characterized and shown to have spectroscopic properties and pigment-protein compositions and stoichiometries similar to those of authentic complexes. Heterologous reconstitutions utilizing pigments and light-harvesting proteins from spinach, pea and Chlamydomonas reinhardtii reveal no evidence of specialized binding sites for the unique C. reinhardtii xanthophyll loroxanthin: lutein and loroxanthin are interchangeable for in vitro reconstitution. Proteins modified by the presence of a transit peptide, phosphorylation, or proteolytic removal of the NH2-terminus could be reconstituted. Evidence suggests that post-translational modification are not responsible for the presence of six electrophoretic variants of C. reinhardtii CP 2. Reconstitution is blocked by iodoacetamide pre-treatment of the apoproteins suggesting a role for cysteine in pigment ligation and/or proper folding of the pigment-protein complex. Finally, no effect of divalent cations on pigment reassembly could be detected.

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.

Freeze-fracture study of plasma membranes in wild type and daunorubicin-resistant Ehrlich ascites tumor and P388 leukemia cells

The plasma membrane is considered to play a major role in the development and maintenance of the multidrug resistance (MDR) phenotype, a role which may in part be mediated by an inducible 170 kD transmembrane protein (P-170). The present freeze-fracture study of plasma membranes of daunorubicin-resistant Ehrlich ascites and P388 leukemia cells demonstrated a significant increase in the density of intramembrane particles (IMP) in the P-face, but not the E-face, of resistant sublines compared with wild type cells. Furthermore, a three-dimensional histogram plot of the diameters of P-face IMPs in Ehrlich ascites tumor cells showed the emergence of a subpopulation of 9 X 11 nm IMPs not found in wild type cells. The size of these IMPs would be consistent with a MW of approximately 340 kD, thus indicating that P-170, shown to be present in both resistant cell lines by Western blot analysis and immunohistochemical staining, exists as a dimer in the plasma membrane. Incubation with the calcium channel blocker verapamil, in concentrations known to inhibit daunorubicin efflux in resistant cells, showed evidence of membrane disturbance in the form of IMP clustering in both wild type and resistant Ehrlich ascites tumor cells. However, incubation with daunorubicin itself did not alter the freeze-fracture morphology of the plasma membranes.

Regulation and expression of the multigene family coding light-harvesting chlorophyll a/b-binding proteins of photosystem II

The current state of knowledge concerning the expression of the nuclear genes that code the light-harvesting chlorophyll a/b-binding polypeptides of photosystem II is presented. This review covers the structure of these genes, the complex multistep pathway involved in their expression, and the environmental and other factors which regulate their expression. Some of the effects of these factors are mediated, at least in part, at the level of transcription, but other effects can be explained only by the existence of multiple posttranscriptional regulatory steps.

Characterization of a full-length petunia cDNA encoding a polypeptide of the light-harvesting complex associated with photosystem I

We have isolated and characterized a full-length cDNA clone (LHCI-15) which specifies a new chlorophyll-binding protein. This protein is associated with the light-harvesting complex of photosystem I (LHCI). The DNA sequence predicts a precursor protein of 270 amino acids, which shares significant homology with the amino acid sequence of another chlorophyll-binding protein; the chlorophyll a/b-binding (Cab) protein of the photosystem II light-harvesting complex (LHCII). There are two extensive regions of homology (at least 45 residues each) which have approximately 50% amino acid sequence identity. These regions coincide with two of the proposed membrane-spanning alpha helices in the Cab proteins of the LHCII and probably include conserved chlorophyll-binding sites. The LHCI-15 cDNA hybridizes to at least 7 genomic EcoRI DNA fragments, which are very closely related at the nucleotide sequence level.