Structural constraints for protein repair in plant photosynthetic membranes

Abstract

The thylakoid membrane system inside plants chloroplasts defines the structural framework for photosynthetic conversion of sunlight into metabolic energy forms (ATP, NADPH + H+). An architectural hallmark of these thylakoid membranes is the tight stacking of part of the membrane into cylindrical flat grana thylakoids, with a diameter of about 500 nm, that are interconnected by unstacked stroma lamellae forming a complex 3D network of alternating grana piles and stroma lamellae. The structural differentiation in the stacked and unstacked thylakoid regions is the basis for a pronounced spatial separation of multisubunit pigment-protein complexes that catalyze energy transformation. The main part of photosystem II (PSII) associated with light-harvesting complex II (LHCII) is concentrated in the grana thylakoids whereas PSI-LHCI and the ATPase complex are excluded from the stacked grana and accumulate in the unstacked thylakoid regions. The fifth protein complex, the cytochrome b6f complex, is assumed to be homogenously distributed. It is important to recognize that this structural arrangement is not static but highly dynamic and responsive to environmental factors like light intensity and quality or temperature. Knowledge about the interplay between dynamic structural features of the intricate thylakoid architecture, and the functionality, regulation, repair and biogenesis of the photosynthetic machinery is essential for understanding the plasticity of energy conversion in plants living in a fluctuating multi-factorial environment.