Measuring Remodeling of the Lipid Environment Surrounding Membrane Proteins with Lipid Exchange and Native Mass Spectrometry.

Due to their crucial biochemical roles, membrane proteins are important drug targets. Although it is clear that lipids can influence membrane protein function, the chemistry of lipid binding remains difficult to study because protein-lipid interactions are polydisperse, competitive, and transient. Furthermore, detergents, which are often used to solubilize membrane proteins in micelles, may disrupt lipid interactions that occur in bilayers. Here, we present two new approaches to quantify protein-lipid interactions in bilayers and understand how membrane proteins remodel their surrounding lipid environment. First, we used mass spectrometry (MS) to measure the exchange of lipids between lipoprotein nanodiscs with and without an embedded membrane protein. Shifts in the lipid distribution toward the membrane protein nanodiscs revealed lipid binding, and titrations allowed measurement of the optimal lipid composition for the membrane protein. Second, we used native or nondenaturing MS to ionize membrane protein nanodiscs with heterogeneous lipids. Ejecting the membrane protein complex with bound lipids in the mass spectrometer revealed enrichment of specific lipids around the membrane protein. Both new approaches showed that the E. coli ammonium transporter AmtB prefers phosphatidylglycerol lipids overall but has a minor affinity for phosphatidylcholine lipids.