• Hans-Erik Åkerlund

Biological membranes are a prerequisite for all living organism. They separate the cell from its environment and sequester different compartments within the cell. Membranes consist of both lipids and proteins. Proteins exchange substances across the membrane and transduce signals from the surroundings and allow the cell to respond in a rational way. Membrane bound proteins located in highly specialized membranes - the mitochondrial inner membrane and the thylakoid membranes of chloroplasts respectively perform cellular energy production. Increasing interest has been focused on the role of the lipids and their dynamic interaction with proteins. The packing of lipids in biological membranes is sensitive to changes caused by temperature, enzymatic modifications as a result of signalling and is particularly sensitive to oxidative stress. For optimal performance it is therefore important that the cells can adapt their membranes to changes in the environment. A membrane that is subjected to large fluctuations in the environment is the thylakoid membrane of higher plants. Plants are subjected to large fluctuations in temperature between day and night, when it is cloudy and sunny, windy or calm. The lipids of the thylakoid membrane are also subjected to the risk of oxidative damage caused by reactive oxygen species (ROS) especially at full sunlight. The thylakoid membrane is therefore a good model system where we expect to find adaptive responses and dynamic interaction between lipids and proteins.

The lipids in the thylakoid membrane are dominated of MGDG (monogalaktosyldiacylglycerol) and DGDG (digalaktosyldiacylglycerol). Most of the fatty acids in these lipids are polyunsaturated. MGDG is a lipid that cannot form lamellar membranes by itself. However, in the presence of DGDG, proteins and xanthophylls it can form lamellar membranes, although with intrinsic curvature stress. The xanthophylls are an important group of carotenoids in the thylakoid membrane. We have earlier studied biochemical aspects of the enzymatic inter conversion between the xanthophylls violaxanthin and zeaxanthin. We have isolated the enzyme VDE (violaxanthin de-epoxidase), characterized the pH and substrate dependence, determined the energy levels in violaxanthin and zeaxanthin by femtosecond spectroscopy, isolated and characterized the gene, made mutated forms of the enzyme and shown that four conserved histidines are important for the binding of the enzyme to the membrane and for the kinetic properties of the enzyme. Recently our interest has focused on the dynamic interplay between lipids and VDE. It has been known for a long time that VDE requires MGDG for its activity, but the reason has not been clear. By systematic investigations we have reach the interesting conclusion that it is not the chemical structure of the head group of the lipid that is important for activity but rather the geometrical form of the whole lipid and how the lipid can be organized that is crucial. We have recently obtained result that show that the degree of intrinsic curvature stress in the membrane has a strong influence on VDE activity. We have also shown that the conversion of violaxanthin to zeaxanthin leads to a decrease of intrinsic curvature stress and that VDE then become inactive. This feedback inhibition thus shows that the process may be crucial for the homeostasis of the membrane.

The main focus of our current work is to study the role of intrinsic curvature stress on the activity and structure of VDE. The activity and binding of other proteins in the thylakoid membrane, and specific proteins in other membranes are also investigated to unravel the role of membrane packing for these proteins.