Artificial membranes as model systems for the investigation of lipid protein interactions in the thylakoid membranes of vascular plants and the green alga Mantoniella squamata
Zusammenfassung der Projektergebnisse
In the present DFG project we studied the lipid protein interactions in vascular plants and the alga Mantoniella squamata with two model proteins: the water-soluble xanthophyll cycle protein violaxanthin de-epoxidase, which binds to the thylakoid membrane upon activation, and the membrane integral light-harvesting complexes. Our results define several new functions for the lipids of the thylakoid membrane. The main lipid of the thylakoids, MGDG, is essential for the de-epoxidation of violaxanthin that is associated with the LHCII of vascular plants or the LHC of M. squamata. According to our data, MGDG forms a lipid shield around the PSII antenna proteins. This MGDG phase solubilises that part of the violaxanthin cycle pigments which is not protein-bound. The shield also marks an attraction site for the enzyme violaxanthin de-epoxidase, thereby guiding it to the place where its substrate, i.e. violaxanthin is preferentially located. Another important result of the present project is that the thylakoid membrane lipids exert different effects on the higher order structure of the light-harvesting complexes of vascular plants and the prasinophycean alga. Negatively charged thylakoid lipids like SQDG and PG lead to a strong disaggregation of the spinach LHCII whereas the neutral galactolipids MGDG and DGDG tend to stabilize the LHCII in its aggregated form. Interestingly, the LHC of M. squamata is unable to form comparable aggregates as the LHCII of vascular plants. It is further characterized by a dramatically decreased sensitivity against the cation Mg2+ which in spinach induces a strong aggregation of the antenna system. Since the Mg2+-dependent LHCII aggregation is discussed as one of the important factors leading to the stacking of thylakoid membranes in the grana regions of vascular plants, the absence LHC aggregation in M. squamata might be responsible for the regular, unstacked arrangement of the thylakoid membranes in the primitive green alga. It is of further interest that the higher order structure of the LHC of M. squamata cannot be modulated by lipids as we have described it for the first time for the spinach LHCII. During the present DFG project we have also analysed in detail the differences in the violaxanthin cycles of vascular plants and prasinophycean algae. The comparison of the amino acid sequences of spinach, Arabidopsis thaliana and the Prasinophyceae Ostreococcus tauri revealed that the algal enzyme shows several amino acid exchanges at positions which have been described as being essential for the correct function of the vascular plant enzyme. According to our data the structure of the prasinophycean violaxanthin de-epoxidase might be modified in such a way that both substrate and co-susbtrate binding, i.e. violaxanthin and ascorbate, are less efficient than in the enzyme of vascular plants. It is further possible that the dimerization of de-epoxidase monomers, which has been postulated to occur at low pH- values, is restricted in the primitive green algae. The modifications of the algal enzyme may result in a decreased efficiency of the conversion of violaxanthin to zeaxanthin, with an increased release of the intermediate reaction product antheraxanthin, as it can be observed in the violaxanthin-antheraxanthin cycle in the intact cells of the Prasinophyceae. During the time course of the DFG project we have also analysed the lipid and fatty acid composition of M. squamata with interesting results. M. squamata thylakoid membranes contain significantly higher amounts of the negatively charged lipid SQDG compared with thylakoids of spinach. It is not unlikely that these high SQDG concentrations lead to extensive membrane areas with a negative surface charge. If these charges cannot be compensated by cations like Mg2+ the repulsion of neighbouring membranes may present another factor which inhibits grana formation in the prasinophycean algae. The native lipids of M. squamata were carefully isolated during the project and native MGDG was compared with commercially available MGDG with respect to an activation of the violaxanthin de-epoxidation reaction. These experiments showed that the lipid isolation yielded active, native MGDG. Based on these results MGDG can now be isolated from different algae, which contain interesting MGDG molecules with long chain fatty acids and a high degree of unsaturation. This native MGDG can then be investigated with respect to its effect on violaxanthin or diadinoxanthin deepoxidation or the structure of algal light-harvesting complexes, like the FCP of diatoms.
Projektbezogene Publikationen (Auswahl)
- Lipids in algae, lichens and mosses. In: N. Murata, H. Wada (Eds.), Lipids in photosynthesis: Essential and regulatory functions, Kluwer Academic Publishers, The Netherlands (2009) pp. 117-137
R. Goss, C. Wilhelm
- Regulation and function of xanthophyll cycle-dependent photoprotection in algae. Photosynth. Res. 106 (2010) 103-122
R. Goss, T. Jakob
- The main thylakoid membrane lipid monogalactosyldiacylglycerol (MGDG) promotes the deepoxidation of violaxanthin associated with the light-harvesting complex of photosystem II (LHCII). Biochim. Biophys. Acta 1797 (2010) 414–424
S. Schaller, D. Latowski, M. Jemiola-Rzeminska, C. Wilhelm, K. Strzalka, R. Goss
- Regulation of LHCII aggregation by different thylakoid membrane lipids. Biochim. Biophys. Acta 1807 (2011) 326–335
S. Schaller, D. Latowski, M. Jemiola-Rzeminska, A. Dawood, C. Wilhelm, K. Strzalka, R. Goss
