The role of the light-harvesting chlorophyll a/b protein complex in chloroplast membrane stacking. Cation-induced aggregation of reconstituted proteoliposomes.

The major intrinsic protein from spinach chloroplast membranes, the light-harvesting chlorophyll a/b-protein complex, contains two distinct polypeptides of Mr 23,500 and 26,000 and 31% lipid by weight, comprising five diacyl lipids and seven chlorophylls, together with some carotenoids, per 26,000-Mr polypeptide. The chlorophyll a/b ratio is 1.1. Low-temperature fluorescence emission spectra of the light-harvesting complex revealed a major peak at 681 nm with a shoulder of variable intensity at 695 nm. The 695-nm emission has been correlated with a progressive aggregation of the complex into two-dimensional, semi-crystalline sheets. To determine the role of the light-harvesting complex in cation-dependent thylakoid stacking, the purified complex has been quantitatively incorporated into liposomes containing the four major chloroplast diacyl lipids using a simple freeze-thaw technique. The proteoliposomes appeared largely as unilamellar vesicles, with diameters between 0.1 and 0.8 micron. Freeze-fracture analysis showed intramembrane particles of 8-10 nm corresponding to the incorporated complex. Both monovalent and divalent cations caused an immediate aggregation of the proteoliposomes, which was reversed at low cation concentrations and was largely inhibited by prior trypsin treatment. Since lipid vesicles themselves showed none of these effects, it is concluded that surface-exposed polypeptide regions of the light-harvesting complex are directly involved in thylakoid stacking in vivo.