Role of LHCII-containing macrodomains in the structure, function and dynamics of grana (vol 26, pg 649, 1999)

In higher plants and green algae two types of thylakoids are distinguished, granum (stacked) and stroma (unstacked) thylakoids. They form a three-dime nsional (3D) network with large lateral heterogeneity: photosystem II (PSII ) and the associated main chlorophyll a/b light-harvesting complex (LHCII) are found predominantly in the stacked region, while PSI and LHCI are locat ed mainly in the unstacked region of the membrane. This picture emerged fro m the discovery of the physical separation of the two photosystems (Boardma n and Anderson 1964). Granal chloroplasts possess significant flexibility, which is essential for optimizing the photosynthetic machinery under variou s environmental conditions. However, our understanding concerning the assem bly, structural dynamics and regulatory functions of grana is far from bein g complete. In this paper we overview the significance of the three-dimensi onal structure of grana in the absorption properties, ionic equilibrations, and in the diffusion of membrane components between the stacked and unstac ked regions. Further, we discuss the role of chiral macrodomains in the gra na. Lateral heterogeneity of thylakoid membranes is proposed to be a conseq uence of the formation of macrodomains constituted of LHCII and PSII; their long range order permits long distance migration of excitation energy, whi ch explains the energetic connectivity of PSII particles. The ability of ma crodomains to undergo light-induced reversible structural changes lends str uctural flexibility to the granum. In purified LHCII, which has also been s hown to form stacked lamellar aggregates with long range chiral order, exci tation energy migrates for large distances; these macroaggregates are also capable of undergoing light-induced reversible structural changes and fluor escence quenching. Hence, some basic properties of grana appear to originat e from its main constituent, the LHCII.