Åke Wieslander

Research Project:
Assembly and properties of membrane lipid bilayers

Address:
e-mail: ake@dbb.su.se 
Phone: (+46)-8-16 2463 
Fax: (+46)-8-15 3679

Research group:
Anja Kunert, Maria Rosén, Tuulia Rämö, Patrik Storm, Malin Wikström

Project description:

Background. A lipid bilayer with a strongly hydrophobic interior is the structural base for all biological membranes and the surrounding environment for most integral ("transmembrane") proteins. In addition, many peripheral ("soluble") membrane proteins bind more or less deep into the polar phosphate and/or sugar headgroups constituting the surface region of the lipid bilayer. For the functions of many proteins, and a structural flexibility of the membranes, a (i) liquid character ("melted state") of the lipid chains is essential. All membranes also have a substantial fraction of negatively charged lipids, yielding a certain (ii) surface charge density (and potential) of the lipid bilayer. Certain common nonbilayer-prone lipids, due to their small polar headgroups, give bilayers a (iii)curvature elastic stress ("frustrated chain packing"), which is large enough to affect protein conformation, bilayer permeability, and potential membrane structural reorganizations. Certain (iv) lipid (and protein) domains and a (v) transmembrane distribution bias (lipids/proteins) are also common. Finally, many proteins are expected to (vi) interact with specific lipids. 
We have been involved establishing points (ii) and (iii) using the small and simple prokaryote Acholeplasma laidlawii as the model; the features have been extended toEscherichia coli plus other bacteria (different lipids/similar principles), and seem to be of general biological significance. 

Our current work concern three related areas:
· How are the surface charge density and curvature stress sensed and metabolically regulated by the cell. The activities of two lipid enzymes from A. laidlawii (peripheral proteins), making a major nonbilayer and a bilayer glycolipid, are governed and set by these interfacial properties. Studies involve (i) structural, (ii) conformational and (iii) binding studies of the (cloned) enzymes in various lipid-enzyme model systems.
· What is the impact of these packing properties on important membrane-associated functions. By the introduction of these enzymes in E. coli lipid mutants certain selected processes such as (i) membrane transport, (ii) stress adaption, (iii) protein secretion, and (iv) cell division are studied as a function of membrane lipid properties.
· Visualization of these principles in other glycolipid-containing membranes.Glycolipids are major constituents in many membranes, e.g. in many pathogenic bacteria, all photosynthetic membranes, and most eukaryotic cell surfaces. With a combination of biochemical, multivariate sequence analysis, and genetic methods, (i) glycolipid enzymes, (ii) regulation, and (iii) bilayer assembly in the minimal human pathogen Mycoplasma pneumoniae and the photosynthetic "model" organismSynechocystis 6803 are investigated.

Collaborators:
We have cooperation with Prof. Michael Sjöström, Chemometrics, Umeå University;
Prof. W. Dowhan, University of Texas at Houston; and Assoc. Prof. Birgitta Norling, Stockholm University.

Selected references: 
· Wieslander, Å. & Rosén, M. (2001) Cell membranes and transport. In "Molecular Biology and Pathogenicity of Mycoplasmas" (S. Razin & R. Herrmann, eds), Kluwer Academic/Plenum Press, in press. 
· Edman, M., Berg, S., Li, L., Wikström, M & Wieslander, Å. (2001) Sequence properties of the monoglucosyldiacylglycerol synthase from Acholeplasma laidlawiimembranes. Recognition of a large group of lipid glycosyltransferases widespread in eubacteria and archea. J. Biol. Chem. 276, 22056-22063.
· Vikström, S., Li, L. & Wieslander, Å. (2000) The nonbilayer/bilayer lipid balance in membranes. Regulatory enzyme in Acholeplasma laidlawii is stimulated by metabolic phosphates, activator phospholipids and double-stranded DNA. J. Biol. Chem. 275, 9296-9302.
· Vikström, S., Li, L., Karlsson, O.P. & Wieslander, Å. (1999) Key role of the diglucosyldiacylglycerol synthase for the bilayer/nonbilayer balance in membranes ofAcholeplasma laidlawii. Biochemistry 38, 5511-5520.
· Edman, M., Jarhede, T., Sjöström, M. & Wieslander, Å. (1999) Different sequence patterns in signal peptides from Mycoplasmas, other Gram-positive bacteria andEscherichia coli: A multivariate analysis. Proteins 35, 195-205.
· Li, L. Karlsson, O.P. & Wieslander, Å. (1997) Activating amphiphiles cause a conformational change of the 1,2-diacylglycerol 3-glucosyltransferase fromAcholeplasma laidlawii membranes. J. Biol. Chem. 272, 29602-29606.
· Karlsson, O.P., Dahlqvist, A., Nordström, S. & Wieslander, Å. (1997) Lipid dependence and basic kinetics of the purified 1,2-diacylglycerol 3-glucosyltransferase from membranes of Acholeplasma laidlawii. J. Biol. Chem. 272, 929-936.
· Karlsson, O.P., Rytömaa, M., Dahlqvist, A., Kinnunen, P. & Wieslander, Å. (1996) Correlation between bilayer lipid dynamics and acticity of the diglucosyl-diacylglycerol synthase from Acholeplasma laidlawii membranes. Biochemistry 35, 10094-10102.
· Sjöström, M., Rännar, S. & Wieslander, Å. (1995) Polypeptide sequence property relationships in Escherichia coli based on auto cross covariances. Chemometrics & Intelligent Lab. Syst. 29, 295-305.

PhD-theses in the group: Christiansson, A. (1981); Rilfors, L. (1982); Clementz, T. (1987); Ruuth, E. (1988); Nyström, S. (1991); Wallbrandt, P. (1992); Dahlqvist, A. (1995); Jarhede, T. (1996); Karlsson, O. (1997);Vikström, S. (1999); Li, L. (2001); Berg, S. (2001); Edman, M. (2001).

Acknowledgements:
The group is supported by grants from the Swedish Science Research Council, The Wenner-Gren Foundation, the Trygger Foundation, and the K.&A. Wallenberg Foundation.