Significance of molecular crowding for the functional protein organisation in grana membranes

Grana membranes are crowded by photosystem (PS) II and its antenna system (LHCII) (~ 75% area occupation). Although it is expected that this extreme packing of proteins causes serious problems for membrane functions its significance for the supramolecular arrangement of PSII and LHCII as well as its effect on the light-harvesting properties and the stability of the PSII supercomplex is poorly understood. We studied the influence of molecular crowding on the antenna organization of PSII by "diluting" the high protein density in grana thylakoids. Therefore BBY membranes were fused with unilamellar liposomes consisting of native lipids. Successful incorporation of extra lipids and the molecular organisation of protein complexes were analysed by density gradient centrifugation, lipid analysis, electron microscopy and fluorescence lifetime imaging microscopy. Time-resolved and steady state chlorophyll a fluorescence spectroscopy at RT and 77K indicate a detachment of LHCII in "diluted" membranes. The detachment correlate almost linear with the lipid/chlorophyll ratio, indicating weak interaction forces between the antenna proteins. In contrast threshold behaviour on the lipid/chlorophyll ratio is apparent for the quantum yield of PSII photochemistry, the reduction in connectivity of PSIIa centres and the PSIIa to b conversion. We postulate a hierarchy in protein interactions which becomes manifest in a differential dependency on protein density. Molecular crowding in grana membranes in combination with interplay between weak and strong interactions could reflect a compromise between the flexibility of the protein arrangement and a high efficiency of light utilization.