Regulation of excitation energy distribution in subchloroplast particles: photosystem I

Small-Angle X-ray Scattering Study of Photosystem I−Detergent Complexes: Implications for Membrane Protein Crystallization

Small-angle X-ray scattering (SAXS) was used to investigate the structure of isolated photosystem I (PSI) complexes stabilized in detergent solution. Two different types of PSI preparation were investigated. In the first preparation, thylakoid membranes were solubilized with Triton X100 and purified by density gradient centrifugation. SAXS data indicated large scattering objects or microphases that can be described as sheets with approximately 68 A thickness and a virtually infinite lateral extension. The observed thickness agreed well with the dimension of a PSI molecule across the thylakoid membrane. In the second preparation, PSI was isolated as before but was further purified by anion exchange chromatography resulting in functional complexes consisting of single PSI units with attached surfactant as evidenced by the particle volume and gyration radius extracted from the SAXS data. Several approaches were used to model the solution conformation of the complex. Three different ellipsoidal modeling approaches, a uniform density ellipsoid of revolution, a triaxial solid ellipsoid, and a core-shell model, found extended structures with dimensions that were not consistent with the PSI crystal structure (Ben-Shem, A.; et al. Nature 2003, 426, 630-635). Additionally, the SAXS data could not be modeled using the crystal structure embedded in a disk of detergent. The final approach considered the possibility that protein was partially unfolded by the detergent. The data were modeled using a "beads-on-a-string" approach that describes detergent micelles associated with the unfolded polypeptide chains. This model reproduced the position and relative amplitude of a peak present in the SAXS data at 0.16 A(-1) but was not consistent with the data at larger length scales. We conclude that the polypeptide subunits at the periphery of the PSI complex were partially unfolded and associated with detergent micelles while the catalytically active core of the PSI complex remained structurally intact. This interpretation of the solution structure of isolated PSI complexes has broader implications for the investigation of the interactions of detergents and protein, especially for crystallization studies.

Variable fluorescence of photosystem I particles and its application to the study of the structure and function of photosystem I

Chlorophyll a fluorescence in Photosystem I (PSI) particles isolated according to the method of Bengis and Nelson [J. Biol. Chem. 252, 4564-4569 (1977)] was found to be dependent on the redox state of both P700 and X (an acceptor on the reducing side of PSI). Addition of dithionite plus neutral red to PSI caused an increase in fluorescence intensity and a shift of the main fluorescence peak from 689 to 674 nm. Addition of electron acceptors such as ferredoxin and methyl viologen decreased the fluorescence yield when added to PSI incubated under anaerobic conditions in the presence of excess dichlorophenol indophenol (DCIPH2). The Km for ferredoxin agreed with that determined from direct measurements of ferredoxin reduction, showing that X is a quencher of fluorescence. P700 was also found to be a quencher of fluorescence, since electron donors such as DCIPH2, TMPD, and plastocyanin decreased fluorescence with Km's nearly identical to those observed for P700+ reduction. Chemical modification of PSI (with ethylene diamine + a water-soluble carbodiimide) to make it positively charged increased the fluorescence yield and shifted the 689-nm peak to 674 nm. The Km's for DCIPH2 and ferredoxin were decreased. In contrast, modification of PSI with succinic anhydride, which increased the net negative charge, increased the Km for ferredoxin. Salts affected the interaction of methyl viologen with PSI. Both anion and cation selectivity were observed. Limited proteolysis increased the Km for both methyl viologen and ferredoxin, indicating that their binding site on PSI was altered. These results suggest that the binding site for ferredoxin is on either the 70- or the 20-kDa subunit of PSI.

Surface-charge related actions of polylysine on thylakoid membranes

Polycation binding to the negatively charged surface of chloroplast thylakoid membranes is known to cause an inhibition of photosystem I activity. It also interferes with the cation-dependent rearrangement of chlorophyll proteins in the thylakoid membrane. It was shown that added anions prevented or reversed the inhibition of photosystem I by polylysine without decreasing its binding to the membranes. Anions also caused a change in the interaction of the chlorophyll proteins in polylysine-treated thylakoids as indicated by an increase in the relative fluorescence intensity from photosystem II. In both cases, the relative effectiveness of the anions tested depended on their valence; for example, the tetravalent species Fe(CN)4-(6) was effective at concentration at least 2 orders of magnitude lower than the divalent species SO2-(4). These results suggest that anions act by screening the positive charge of the polylysine-coated membrane surface. Measurements of the response of the anionic fluorescent probe 1-anilinonapthalene-8-sulfonate to an addition of anions to polylysine-treated thylakoids supported this contention. It was concluded that the action of polylysine on photosystem I and on the chlorophyll proteins is mediated by changes of the electrical properties of the thylakoid membrane and may not involve a direct binding of the polycation to the affected membrane proteins.

Membrane-surface electric properties of triton-fractionated spinach subchloroplast fragments

Surface charge density of subchloroplast fragments fractionated from spinach by Triton X-100 treatment was estimated from cation-induced quenching of chlorophyll fluorescence, with the premise that the fluorescence yield is dependent on the surface electric potential of the preparations. Application of the Gouy-Chapman theory of diffuse double layer to the subchloroplast preparations, or treating the surface of the preparations under electric charge regulation conditions yielded a result suggesting the Photosystem II reaction-center preparation (TSF-IIa) to be more negatively charged than the Photosystem I reaction-center preparation (TSF-I). Isoelectric points of the subchloroplast fragments were determined by measuring 90 degrees light scattering and more directly by gel isoelectric focusing. Isoelectric points of TSF-I and -IIa were estimated to be 4.8 and 4.0 from light-scattering experiments, and 4.5 and 4.1 from gel electrophoresis, respectively. The TSF-II preparation that contains both a light-harvesting complex and the reaction-center (core) complex showed a small cation-induced quenching of chlorophyll fluorescence. This fluorescence quenching may be ascribed mostly to the regulation of energy transfer in the preparation (Yamamoto, Y. and Ke, B. (1980) Biochim, Biophys. Acta 592, 296-302). Furthermore, the TSF-II preparation showed a broad and indefinite peak in light scattering in the pH range 3-8, suggesting that the complex probably carries a small amount of charge in this pH range. The physiological role of the membrane surface charge of the subchloroplast preparations in membrane structure and cation regulated processes in chloroplast is discussed.

Use of chemical modification to study the relationship between activity and net protein charge of the photosystem I core complex

The net charge on the photosystem I core complex of Shiozawa et al. (Shiozawa, J. A., Alberte, R. S., & Thornber, J. P. (1974) Arch. Biochem. Biophys. 165, 388-397) has been altered by using a water-soluble carbodiimide to form an amide bond between protein carboxyl groups and an amino group of ethylenediamine. This process replaces negatively charged carboxyl groups with positively charged free amino groups. Six hundred moles of ethylenediamine was incorporated per mole of P700 reaction center. The modification of the complex shifted the isoelectric pH of photosystem I from 5.0 to 9.5 without affecting the total amount of P700. The modified complex exhibited an increase in energy transfer from light-harvesting chlorophyll alpha molecules to the reaction center. This phenomenon has been previously observed upon addition of Mg2+ ions to the complex (Gross, E. L., & Grenier, J. (1978) Arch. Biochem. Biophys. 187, 387-398). The modification replaces the divalent cation requirement for electron donation to P700 by plastocyanin and lowered the Km for plastocyanin binding to 2.0 microM, compared to 32 microM for control photosystem I in the presence of Mg2+. In addition, the modification lowered the Km for the negatively charged electron donors dichlorophenolindophenol and ascorbate. These results suggest that changing the charge on the photosystem I complex from negative to positive stimulates both light utilization and electron transfer from electron donors to P700. We suggest that cation regulation of photosystem I activity occurs by a process in which cations alter the charge of the local environment around the complex.

Regulation of excitation energy distribution in photosystem-II fragments by magnesium ions

Fluorescence characteristics of Photosystem-II subchloroplasts (TSF-II and TSF-IIa) fractionated by Triton X-100 treatment were studied in relation to cation-induced regulation of excitation-energy distribution within subchloroplast fragments. Absorption spectra and fluorescence-emission spectra at 77 K showed that TSF-II contains the light-harvesting chlorophyll-protein complex in addition to the reaction-center complex, which is present alone in TSF-IIa. Mg2+ increased the ratio of F695nm to F685nm in the fluorescence-emission spectrum of TSF-II particles at 77 K, but had no effect on TSF-IIa particles. Mg2+ also induced a quenching of chlorophyll fluorescence at room temperature in TSF-II, an effect that was insensitive to the presence of DCMU. The DCMU-insensitive fluorescence quenching was not observed in the TSF-IIa preparation. These results suggest an existence of cation-induced regulation of excitation-energy transfer in TSF-II preparations. Presence of antenna chlorophyll molecules alone does not seem to be sufficient for observing energy-transfer regulation by cations in Photosystem-II preparations.