Detergent exchange from lipid nanoparticles into detergent micelles unlocks a tool for biochemical and kinetic characterization of membrane proteins

Bacterial membrane proteins make up ~30% of the prokaryotic genome and play key roles in infection and virulence. Membrane protein chemistry has advanced in recent years, including purification strategies that mimic "native-like" lipid environments, such as lipid nanoparticles, amphipols, and nanodiscs. The use of styrene maleic acid co-polymers to form a lipid nanoparticle has become increasingly common in membrane protein purification, especially for proteins which are not amenable to detergent extraction from the cellular membrane fraction. Yet, for some biochemical and biophysical methods it is preferable to use detergent-solubilized protein. Here we show a general exchange method to transfer membrane proteins from lipid nanoparticles to detergent micelles while retaining protein fold, homogeneity and function. Conditions were first optimized for co-polymer dispersion and recovery into detergents, and analytical methods employed to assess activity and quality of detergent-solubilized proteins. Twelve protein targets were purified in co-polymer based on a 16-polymer screen. This selection was followed by an eight-detergent screen in the presence of calcium ions for optimal dissolution of the nanoparticle, producing detergent-stabilized protein. In all membrane proteins assessed, homogeneity and folding were retained from the initial purification in lipid nanoparticles through the detergent-exchange protocol. For membrane enzymes that have proven to be experimentally intractable once detergent solubilized, we were able to observe catalytic activity using the detergent-exchanged material. The use of this protocol to purify membrane proteins provides great versatility for biochemical and kinetic characterization than was previously accessible.