A highly purified light-harvesting pigment-protein complex (LHC) was obtained by fractionation of cation-depleted chloroplast membranes using the nonionic detergent, Triton X-100. The isolated LHC had a chlorophyll a b ratio of 1.2 and exhibited no photochemical activity. SDS-polyacrylamide gel electrophoresis of the LHC revealed three polypeptides in the molecular weight classes of 23, 25, and 30 × 103. Antibodies were prepared against the LHC and their specificity was established. The effect of the α-LHC (antibodies to LHC) on salt-mediated changes in PS I and PS II photochemistry, Chl α fluorescence inductions, and 77 °K fluorescence emission spectra was investigated. The results show that: (i) The Mg2+-induced 20% decrease in photosystem I (PS I) quantum yield observed in control chloroplasts was blocked by the presence of the α-LHC antibody, (ii) The Mg2+-induced 70% increase in photosystem II (PS II) quantum yield of control chloroplasts was reduced 35% for plastids in the presence of α-LHC antibody, (iii) The Mg2+-induced increase in room-temperature variable fluorescence was reduced 60% by α-LHC antibody, (iv) The Mg2+-induced increase in the F685 F730 emission peak ratio at 77 °K was inhibited 50% in the presence of α-LHC antibody. These results provide direct evidence for the involvement of the light-harvesting complex in cation regulation of energy redistribution between the photosystems. The fact that the α-LHC antibody does not fully block Mg2+-induced PS II increases or chlorophyll fluorescence increases supports the concept that Mg2+ has two mechanisms of action: one effect on energy distribution and a second direct effect on photosystem II centers.
ASJC Scopus subject areas
- Molecular Biology