Biochemical characterization of photosystem I-associated light-harvesting complexes I and II isolated from state 2 cells of Chlamydomonas reinhardtii.

Two photosystems, PSI and PSII, drive electron transfer in series for oxygenic photosynthesis using light energy. To balance the activity of the two photosystems under varying light conditions, mobile antenna complexes, light-harvesting complex IIs (LHCIIs), shuttle between the two photosystems during state transitions. PSI forms a complex consisting of PSI core and its peripheral light-harvesting complex (LHCI) in plants and algae. In a previous study, we isolated a PSI-LHCI-LHCII supercomplex containing both LHCI and LHCII from state 2 cells of Chlamydomonas reinhardtii. In the present study, we isolated a PSI-LHCI-LHCII supercomplex associating with more LHCII complexes under a further optimized protocol. We determined its antenna size by three independent methods and revealed that the associated LHCIIs increased the antenna size by about 70 Chls and transferred light energy to the PSI core. Uniform labeling of total cellular proteins with (14)C indicated that the PSI-LHCI-LHCII supercomplex contains 1.85 copies of LhcbM5 and CP29 and 1.29 copies of CP26. PSI-LHCI-LHCII also stably bound 0.4 copy of ferredoxin-NADP(+) oxidoreductase (FNR) that catalyzes light-induced electron transfer from PSI to NADP(+) in the presence of ferredoxin. We discuss the possible organization of these LHCIIs in the PSI-LHCI-LHCII supercomplex.