Abstract
The most enigmatic feature of mature thylakoids of Angiosperms is the presence of piles of membranous discs forming the cylindrical structures known as grana. Although some models aim to elucidate their formation, until now the mechanism governing the architecture of thylakoid stacks remains obscure. In this work a new model is presented aiming to explain the way thylakoids stack. In comparison with previous models, this model proposes a dynamic mechanism for the rapid selfassembly of thylakoid stacks and their subsequent disassembly under the influence of a variety of physicochemical factors and is consistent with the evolutionary origin of these membranes and their ontogenetic continuity. The model proposes that, under the influence of attractive electrostatic forces, the membranes come closer in a parallel alignment and the photosystem II/light harvesting complexes migrate laterally forming circular aggregates. Finally the thylakoids rotate around the vertical axis of the superimposed aggregates, under the action of a torque.
Highlights
Mature thylakoid membranes of green plants and some green algae are differentiated into unstacked or stroma lamellae and stacked or appressed thylakoids or grana
Thylakoids present structural heterogeneity; the main fraction of photosystem II (PSII) and its light harvesting complex (LHCII) reside mainly in the stacked thylakoids, whereas photosystem I (PSI) and its LHCI, as well as the ATP synthase are predominantly concentrated in stroma lamellae.[2]
The structure of pea LHCII trimer and its charge distribution on the stromal surface has been presented by Standfuss and colleagues,[11] who proposed that electrostatic attractive forces between LHCII of opposite membranes play a major role in the cohesion of grana
Summary
Mature thylakoid membranes of green plants and some green algae are differentiated into unstacked or stroma lamellae and stacked or appressed thylakoids or grana. It is generally believed that, in green plants, stacked and unstacked thylakoids form a continuous membrane that encloses a single compartment, the lumen, and that stacked thylakoids are cylindrical structures with a diameter of 300-600 nm.[1,2,3] Thylakoids present structural heterogeneity; the main fraction of photosystem II (PSII) and its light harvesting complex (LHCII) reside mainly in the stacked thylakoids, whereas photosystem I (PSI) and its LHCI, as well as the ATP synthase are predominantly concentrated in stroma lamellae.[2] Under certain conditions, thylakoids can undergo changes from the stacked to the unstacked state These changes are rapid, dynamic and reversible. The above mentioned models explain neither the molecular basis nor the involvement of the physicochemical forces on the formation of grana and, the mechanism governing the spatial organization of the thylakoid membranes remains obscure None of these models provides proof for the evolutionary origin of these cylindrical structures or their ontogenetic continuity.
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