Assessing Interactions between a Polytopic Membrane Protein and Lipid Bilayers Using Differential Scanning Calorimetry and Solid-State NMR

It is known that the lipid composition within a cellular membrane can influence membrane protein structure and function. In this Article, we investigated how structural changes to a membrane protein upon substrate binding can impact the lipid bilayer. To carry out this study, we reconstituted the secondary active drug transporter EmrE into a variety of phospholipid bilayers varying in headgroup and chain length and carried out differential scanning calorimetry (DSC) and solid-state NMR experiments. The DSC results revealed a difference in cooperativity of the lipid phase transition for drug-free EmrE protonated at glutamic acid 14 (i.e., proton-loaded form) and the tetraphenylphosphonium (TPP⁺) bound form of the protein (i.e., drug-loaded form). To complement these findings, we acquired magic-angle-spinning (MAS) spectra in the presence and absence of TPP⁺ by directly probing the phospholipid headgroup using ³¹P NMR. These spectra showed a reduction in lipid line widths around the main phase transition for samples where EmrE was bound to TPP⁺ compared to the drug free form. Finally, we collected oriented solid-state NMR spectra on isotopically enriched EmrE that displayed chemical shift perturbations to both transmembrane and loop residues upon TPP⁺ binding. All of these results prompt us to propose a mechanism whereby substrate-induced changes to the structural dynamics of EmrE alters the surrounding lipids within the bilayer.