Arrangement of photosystem II supercomplexes in crystalline macrodomains within the thylakoid membrane of green plant chloroplasts

The chloroplast thylakoid membrane of green plants is organized in stacked grana membranes and unstacked stroma membranes. We investigated the structu ral organization of Photosystem II (PSII) in paired grana membrane fragment s by transmission electron microscopy. The membrane fragments were obtained by a short treatment of thylakoid membranes with the mild detergent n-dode cyl-alpha,D-maltoside and are thought to reflect the grana membranes in a n ative state. The membranes frequently show crystalline macrodomains in whic h PSII is organized in rows spaced by either 26.3 nm (large-spaced crystals ) or 23 nm (small-spaced crystals). The small-spaced crystals are less comm on but better ordered. Image analysis of the crystals by an aperiodic appro ach revealed the precise positions of the core parts of PSII in the lattice s, as well as features of the peripheral light-harvesting antenna. Together , they indicate that the so-called C2S2 and C2S2M supercomplexes form the b asic motifs of the small-spaced and large-spaced crystals, respectively. An analysis of a pair of membranes with a well-ordered large-spaced crystal r eveals that many PSII complexes in one layer face only light-harvesting com plexes (LHCII) in the other layer. The implications of this type of organiz ation for the efficient transfer of excitation energy from LHCII to PSII an d for the stacking of grana membranes are discussed. (C) 2000 Academic Pres s.