Structure and Orientation of Transmembrane Peptides in Model Membrane Systems as Studied by Solid-State 2H NMR and Molecular Dynamics

Abstract

Protein-lipid interactions play an essential role in influencing the structure and activity of membrane proteins. In particular, the tilting properties of transmembrane segments of proteins, which adopt in the most cases an alpha-helical conformation, are very important for the function of proteins and can be influenced by changes in the lipid composition. The study of lipid-protein interactions by using natural membrane proteins in lipid bilayers is however very complicated. Therefore, model systems composed of designed peptides and synthetic lipids are more suitable. The present thesis exemplifies such an approach where both the peptide and lipid composition were systematically varied with the use of the so-called WALP and analogous peptides as mimic of the membrane-spanning parts of proteins in bilayers composed of single-species lipids. A new solid-state 2H NMR approach, called GALA (geometric analysis of labeled alanines) is developed for studying how membrane parameters like the hydrophobic bilayer thickness, the lipid-peptide interfacial interactions and the packing properties of the membrane can influence the tilt and azimuthal orientations, and the dynamics of transmembrane segments of proteins. In chapter 2, it was shown that the tilt angle of WALP23 increases slightly but systematically with increasing hydrophobic mismatch in lipid bilayers of decreasing membrane thickness. Interestingly, the direction in which WALP23 is tilted (i.e. the azimuthal or rotation angle) is constant in all studied PC-bilayers. The GALA approach was also developed to non-oriented samples, which mimic better biological membranes than macroscopically oriented samples, enable better control of environmental conditions such as pH and salt concentration, and allow studies with poorly orientable lipids. In chapter 3, the results show by comparing the properties of WALP23 and of its lysine-flanked analog KALP23 that the nature of anchoring residues influences not only the tilt angle but also especially the azimuthal angle. The use of poly-leucine analogs of WALP23 and KALP23 (WLP23 and KLP23) show that the hydrophobicity of the central part of the peptide neither influences the tilt angle, nor the azimuthal angle. In chapter 4, it was shown that changing the properties of the membrane-water interfacial region by adding the anesthetic molecule 2,2,2-trifluoroethanol to the model membranes affects the orientational behaviour of the model peptides depending on the nature of the interfacial anchoring residues. The alcohol interferes with the interfacial interactions of tryptophan-flanked peptides like WALP23, whereas lysine equivalents are insensitive to its presence. The molecular dynamics simulations presented in chapter 5 help understanding the mechanisms by which WALP23 is interacts with the surrounding lipids. The results emphasize the importance of the secondary structure on the orientation of WALP23 in lipid bilayers. In general, the length of simulations is an important issue for giving a representative view on the lipid-peptide interactions involved. Also, some insight is gained on the interfacial interactions and energetic aspects that influence the tilt and rotation angles and the secondary structure. Chapter 6 summarizes the results presented throughout the thesis and suggests future perspectives for the development of the methodology to membrane proteins and antibiotics.