Lipid- and Cholesterol-Mediated Timescale-Specific Modulation of Membrane Protein Dynamics

Abstract

Membrane protein function fundamentally depends on the surrounding lipid bilayer. In the past few years, it has become clear that this fundamental relationship is intricate and multilayered, going far beyond simple hydrophobic interactions (e.g. Autzen et al, Science, 2018). It has also become clear that mechanisms of membrane protein activation by temperature exist, likely involving lipid dynamics as a regulatory factor (e.g. Gao et al, Nature, 2016). Hence, atomic insights into lipid-mediated modulation of membrane protein dynamics would provide new insights with the potential to fundamentally extend our understanding on lipid-protein interdependencies. Although highly anticipated, such insights remained elusive due to its intrinsic difficulty. To elucidate the impact of lipid dynamics on a membrane protein, we reconstituted the outer membrane protein X (OmpX) into a saturated, unsaturated and cholesterol-containing lipid-bilayer using the nanodisc technology. We then recorded timescale-specific NMR relaxation experiments with atomic resolution of both the membrane protein and lipids within the same nanodisc. The relaxation data revealed that lipid order, modified either biochemically or biophysically, changes the dynamics of the immersed membrane protein in a specific and timescale-dependent manner. A temperature-dependent dynamics analysis furthermore suggests a direct coupling between lipid and protein dynamics in the picosecond, microsecond and millisecond timescale, caused by the lipid's trans-gauche isomerization, the rotational diffusion of lipids and the fluidity of the lipid phase, respectively. These observations evidence a direct modulatory capability of the membrane to regulate protein function through lipid dynamics. Strikingly, the available timescales for such coupling covers at least 6 orders of magnitude, providing a substantially larger repertoire to regulate functions of immersed proteins, as compared to its aqueous counterpart.