Role of LHCII-containing macrodomains in the structure, function and dynamics of grana

Abstract

In higher plants and green algae two types of thylakoids are distinguished, granum (stacked) and stroma (unstacked) thylakoids. They form a three-dimensional (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 located mainly in the unstacked region of the membrane. This picture emerged from the discovery of the physical separation of the two photosystems (Boardman and Anderson 1964). Granal chloroplasts possess significant flexibility, which is essential for optimizing the photosynthetic machinery under various environmental conditions. However, our understanding concerning the assembly, structural dynamics and regulatory functions of grana is far from being complete. In this paper we overview the significance of the three-dimensional structure of grana in the absorption properties, ionic equilibrations, and in the diffusion of membrane components between the stacked and unstacked regions. Further, we discuss the role of chiral macrodomains in the grana. Lateral heterogeneity of thylakoid membranes is proposed to be a consequence of the formation of macrodomains constituted of LHCII and PSII; their long range order permits long distance migration of excitation energy, which explains the energetic connectivity of PSII particles. The ability of macrodomains to undergo light-induced reversible structural changes lends structural flexibility to the granum. In purified LHCII, which has also been shown to form stacked lamellar aggregates with long range chiral order, excitation energy migrates for large distances; these macroaggregates are also capable of undergoing light-induced reversible structural changes and fluorescence quenching. Hence, some basic properties of grana appear to originate from its main constituent, the LHCII.