Understanding Structural Flexibility of Photosynthetic Membranes

Stefanie Tietz

The survival and fitness of photosynthetic organisms is critically dependent on the flexible response of the thylakoid membrane, harboring the photosynthetic machinery, and environmental changes. The first part of this work shows that under high light stress, architectural changes are crucial to ensure an efficient photosystem II (PSII) repair. This is realized by a multistep process through lateral shrinkage of the grana diameter and increasing protein mobility. For this to work, high light triggers an architectural switch in the thylakoid membrane advantageous for quick protein repair. Furthermore, the protein kinase mutants stn8and stn7/8 demonstrate that protein phosphorylations are involved in this switch. Another structural change that occurs under unfavorable conditions has been shown to be the formation of semicrystalline protein arrays. By using a fatty acid desaturase (fad5) mutant with constitutive arrays as a model, we were able to decipher the functional implications of arrays in facilitating molecular diffusion of photosynthetic components through crowded thylakoid membranes. We showed that facilitated diffusion of lipid-like molecules had a direct impact on plastoquinone-dependent electron transfer and photoprotective non-photochemical quenching (xanthophylls). Unfortunately, it also significantly impaired mobility of damaged PSII within the arrays due to tight protein packing. Finally, in an attempt to identify factors influencing structural changes during stress, we studied the native lipid environment of proteins in the membrane. By embedding the light-harvesting complex of PSII (LHCII) into liposomes with a defined lipid content, we were able to study changes to its structure and function due to a non-bilayer lipid monogalactosyl-diacylglycerol (MGDG). LHCII was shown to switch from a light-harvesting to an energy-dissipative state when a natural content of the non-bilayer lipid MGDG is present when compared to liposomes without MGDG. Both circular dichroism and Raman spectroscopy have further indicated a conformational change to the LHCII-bound xanthophyll neoxanthin between different concentrations of MGDG. Overall, this work demonstrates that the photosynthetic membrane shows a high flexibility to different environmental conditions allowing it to adapt to a rapidly changing environment and still efficiently work on photosynthetic energy conversion.

Understanding Structural Flexibility of Photosynthetic Membranes

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