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|Title:||The Structure and Function of Chlorophyll-Proteins in Photosystem I and The Light Harvesting Complex of Photosystem Ii|
|Author(s):||Mullet, John Emerson|
|Department / Program:||Biology|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Abstract:||The functional units of chloroplast membranes which mediate photosynthetic electron transport from water to NADP are structurally organized into integral complexes. Two of these membrane-bound complexes, Photosystem I (PS I) and Photosystem II (PS II), consist in part of chlorophyll proteins. The chlorophyll-protein function to absorb incident light energy and to transfer absorbed energy to photochemical reaction centers where charge separation occurs. The structure and function of the chlorophyll-proteins of PS I and the light harvesting complex of PS II have been analyzed to provide information concerning the organization of pigmented proteins in photosynthetic membranes.
Photosystem I complexes were isolated by a new technique which allowed purification of PS I which retained characteristics attributed to PS I in vivo. This purified preparation of PS I was found to consist of 11 polypeptides (6-68 kilodaltons), 110 chlorophyll per P(,700), and to retain long wavelength chlorophyll a which absorbs at 700-710 nm and emits fluorescence at 730-736nm at 77K. Chlorophyll-protein complexes of PS I were reconstituted in lipid vesicles and were found by freeze-fracture analysis to be structurally organized into particles 106 (')(ANGSTROM) in diameter.
The purified PS I complexes were further fractionated by detergent treatment and PS I complexes which were depleted of chlorophyll-proteins were isolated. Depletion of 40 chlorophyll per P(,700) occurred concomitant with the loss of polypeptides of 20-25 kilodaltons and long wavelength fluorescence emission. These results led to the hypothesis that PS I contained a peripheral light harvesting chlorophyll-protein complex which was characterized by the presence of long wavelength fluorescence emission and polypeptides of 20-25 kilodaltons. This hypothesis was confirmed by studies of chlorophyll proteins which are incorporated during chloroplast development, analysis of a chlorophyll b-less barley mutant and of a mutant which lacked P700 and by fractionation of PS I with anionic detergents. Direct verification of the existence of a peripheral antennae chlorophyll-protein of PS I was obtained by separation of PS I into a core complex which contains P(,700) and a chlorophyll-protein complex which lacks P(,700), but retains polypeptides of 20-25 kilodaltons and exhibits 77 K fluorescence emission at 730-736nm.
The major chlorophyll-protein of chloroplast membranes which is structurally organized into light harvesting complexes associated with PS II was isolated and characterized and its role in thylakoid adhesion tested. The chlorophyll-protein complex, termed LHC-II, was purified by detergent treatment and sucrose gradient centrifugation. The complex consisted of 3-4 polypeptides of 25-29 kilodaltons and associated chlorophyll a and b. LHC-II particles were analyzed by freeze-fracture techniques and were found to be organized into particles of 80 (')(ANGSTROM) in diameter.
The LHC-II complexes were incorporated into lipid vesicles; this preparation was used to demonstrate the involvement of LHC-II in cross-membrane adhesion between thylakoid membranes. Adhesion in LHC-II preparations was dependent on cations and sensitive to trypsin treatment. Trypsin treatment removed peptides were isolated, analyzed and sequenced. The tryptic peptides contained a site of phosphorylation and contained significant positive charge. These structural features of LHC-II formed the basis of a proposed contact mechanism for thylakoid adhesion.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1980.
|Date Available in IDEALS:||2015-05-14|