Modulation of Membrane Protein Dynamics in Lipid Bilayers

Lipids and sterols are essential building blocks for cellular membranes and compartments and their primary function is to divide space. They subdivide the extracellular from the intracellular space and also within the intracellular space organelles such as the mitochondria, the endoplasmic reticulum or the Golgi apparatus possessing membranes with specific properties. In addition to the partitioning of space, membranes have a separation capacity based on low permeability, which allows generating a concentration gradient between interior and exterior space. Different types of proteins within the membrane enable the specific transport of ions, fatty acids, sterols or amino acids through the membrane and in addition to the transport function of particles membrane proteins enable cellular potentials, signal transduction, secretion of proteins or synthesis of ATP. A large diversity of lipids allows cellular membranes to provide the optimal lipid environment tailored to the function of the membrane protein, by variations of the lipids acyl chain length, the degree of unsaturation and the type of head group. The majority of cellular plasma membranes are composed out of an asymmetric bilayer consisting of saturated lipids with a high lipid transition temperature in the outer lipid leaflet and mono- or polyunsaturated lipids with a low lipid transition temperature in the inner lipid leaflet increasing the complexity of lipid membranes even more. To investigate the structural and dynamic influence of the lipid environment on a specific membrane protein using NMR spectroscopy, the membrane protein needs to be first extracted from its natural lipid bilayer environment followed by reconstitution into a membrane mimetic. For membrane mimetics often detergent micelles, lipid bicelles or lipid nanodisc are used. A comparison of the dynamical differences for outer membrane protein X (OmpX) reconstituted in these three membrane mimetics will be presented in Chapter 2 of this thesis. There, we have shown that in presence of lipids a pronounced dynamic variability in the beginning and ends of the beta-sheet regions of the transmembrane protein in the pico- to the nanosecond and as well in the micro- to millisecond time regime was observed. This dynamic variability seems to be less pronounced for OmpX in detergent micelles. Therefore the dynamic difference compared to the lipid environment may have a regulatory function for membrane protein activity, perhaps induced by lipid composition and dynamics variations in the membrane protein environment. Understanding the biological lipid diversity in more detail, the effect of lipid transition temperature on the spectral quality of OmpX in the nanodisc was investigated using solid-state and solution NMR, which we present in Chapter 3. Therefore, the acyl chain length of the saturated lipids is altered, resulting in an increase of the lipid transition temperature. In both solid-state and solution NMR spectroscopy, the highest spectral resolution was observed above the lipid transition…