The Structure and Function of Chlorophyll-Proteins in Photosystem I and The Light Harvesting Complex of Photosystem Ii
Mullet, John Emerson
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Permalink
https://hdl.handle.net/2142/77579
Description
Title
The Structure and Function of Chlorophyll-Proteins in Photosystem I and The Light Harvesting Complex of Photosystem Ii
Author(s)
Mullet, John Emerson
Issue Date
1980
Department of Study
Biology
Discipline
Biology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, General
Language
eng
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.
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