Az Eszterházy Károly Tanárképző Főiskola Tudományos Közleményei. 2004. Sectio Biologiae. (Acta Academiae Paedagogicae Agriensis : Nova series ; Tom. 25)

Mustárdy, L.: Tree-dimensional Organization of Thylakoid System in Granal Chloroplasts and its Functional Role

Three Dimensional Organisation of Thylakoid System. 101 induce such variations, and thus the membrane system must therefore be considered to exhibit substantial structural flexibility. Functions based on ultrastructure More than 30 years ago Boardman and Anderson (1964) have revealed that the differentiation of thylakoids into granum and stroma membrane regions reflects a compositional and functional difference. The two photosystems are spatially separated: photosystem II (PSII) and its main chlorophyll a/b light-harvesting complex, LHCII, are found predominantly in the stacked membranes; this region is largely deficient in photosystem I (PSI) and LHCI, which are enriched in the stroma membranes (Andersson and Anderson, 1980). Separation of the two pigment systems is probably important in preventing unregulated excitation energy flow between the two photosystems (Anderson and Andersson, 1988; Trissl and Wilhelm 1993). Without this, PSI, which is much faster than PSII, would disturb the balance of the energy distribution between the two photosystems. It is known that LHCII has large self-aggregation ability. Indeed, LHCII has been shown to stabilize the granum ultrastructure, and to participate in the cation-mediated stacking of the membranes (Arntzen, 1978; Duniec et al., 1981; Barber, 1982). The abundance of LHCII in the granum suggests that these antenna complexes also play a structural role to separate the two photosystems. These light-harvesting complexes have also been shown to be involved, via electrostatic and osmotic forces, in the lateral organization of the membranes (Garab et al. 1991). Separation of the two photosystems (the LHCII-PSII and LHCI-PSI supercomplexes) between stacked and unstacked regions evidently requires a lateral force, such as that which governs the formation of LHCII-containing macrodomains (Garab and Mustárdy, 1999). The fact that in all LHCII-containing chloroplasts the two photosystems are segregated between the granum and stroma membranes suggests that LHCII may be involved not only in stacking but also in the lateral separation of PSII and PSI. The helical arrangement of the stroma thylakoids around the cylindrical granum stacks has several functions. The continuity of the membrane is obviously warranted within the same granum-stroma assembly. The fusion of two (or more) stroma membranes at the edges ensures the continuity between the two types of membrane for the entire chloroplast. The continuum of the membrane system is evidently important for the diffusion of mobile components between the two regions (like plastocyanin). The 3D organization of the membrane may have special significance in the PSII repair cycle, during which the damaged particles are transferred from the granum to the stroma membranes where they are repaired and reassembled and then transferred back to the stacked region (cf. Melis 1991; Russel et al.