Supramolecular Assembly and Function of Subunits Associated with the Chlorophylla-bLight-Harvesting Complex II (LHC II) in Soybean Chloroplasts

摘要

The chlorophyll (Chl)a-blight harvesting complex II (LHC II) contains more than 80of the light-harvesting pigments of photosystem II (PS II) in chloroplasts. The supramolecular assembly and function of this auxiliary antenna system was investigated in Chi b-deficient and Chi b-less mutant chloroplasts from soybean and barley plants, and in their wild-type counterparts. Four distinct LHC II polypeptides were resolved by SDS-PAGE (subunits a, b, c and d), having apparent molecular masses of 29, 28, 27.2 and 26.8 kDa, respectively. The analysis of LHC II subunit composition in different developmental stages of the PS II unit in soybean (3>Chla/Chlbb>6), indicated the association of specific subunits with the LHC H-inner and LHC II-peripheral in the chloroplast. The amount of subunit a in PS II was constant over a broad range of Chla/Chlbratios, suggesting that this subunit is closely associated with the PS II-core complex. Subunit d also appeared to be constant over a wide range of Chla/Chlbratios, suggesting close association with the LHC II-inner. The PS II content in subunits b and c increased with the PS II antenna development in soybean but the ratio of b/c remained constant in all developmental stages and equal to 2 :1. Subunit a was present in the Chlb-less chlorina f2 mutant of barley grown under continuous illumination but was absent under intermittent illumination. The results suggest that each subunit binds 13-15 Chl molecules. A working hypothesis is presented on the PS II antenna development and LHC II subunit composition in soybean chloroplasts.