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 structural organization of Photosystem II (PSII) in paired grana membrane fragments by transmission electron microscopy. The membrane fragments were obtained by a short treatment of thylakoid membranes with the mild detergent n-dodecyl-α,d-maltoside and are thought to reflect the grana membranes in a native state. The membranes frequently show crystalline macrodomains in which 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 common but better ordered. Image analysis of the crystals by an aperiodic approach revealed the precise positions of the core parts of PSII in the lattices, as well as features of the peripheral light-harvesting antenna. Together, they indicate that the so-called C2S2 and C2S2M supercomplexes form the basic motifs of the small-spaced and large-spaced crystals, respectively. An analysis of a pair of membranes with a well-ordered large-spaced crystal reveals that many PSII complexes in one layer face only light-harvesting complexes (LHCII) in the other layer. The implications of this type of organization for the efficient transfer of excitation energy from LHCII to PSII and for the stacking of grana membranes are discussed.