Heterogeneity of the main light-harvesting chlorophyll a/b–protein complex of photosystem II (LHCII) at the level of trimeric subunits

To study organization of the main light-harvesting chlorophyll a/b–protein complex of photosystem II (LHCII) from spinach thylakoid membranes at the level of trimeric subcomplexes, we have applied non-denaturing isoelectric focusing (ndIEF) in vertical, slab polyacrylamide gels. When analyzed by two consecutive ndIEF/electroelution runs, spinach BBY membrane preparations (PSIIα-enriched, stacked thylakoid membranes) were resolved into nine fractions of 100% purity, labelled 1–9 in order of decreasing pI values. Seven of these fractions (3–9) were shown by absorption spectroscopy to stand for LHCII subcomplexes. The subcomplexes were established — by monitoring their circular dichroism spectra and comparing them to the spectra of native LHCII trimers and monomers — to be structurally intact trimers. The analysis of polypeptide composition of the subcomplexes in terms of apparent molecular masses and Lhcb genes’ products led us to the conclusion that each of the subcomplexes might be a mixed population of closely similar individual trimers, comprising of permutations of Lhcb1 and Lhcb2 (subcomplexes 3–7) or Lhcb1, Lhcb2 and Lhcb3 (subcomplexes 8 and 9).