Structure and function of membrane proteins encapsulated in a polymer-bound lipid bilayer.

New technologies for the purification of stable membrane proteins have emerged in recent years, in particular methods that allow the preparation of membrane proteins with their native lipid environment. Here, we look at the progress achieved with the use of styrene-maleic acid copolymers (SMA) which are able to insert into biological membranes forming nanoparticles containing membrane proteins and lipids. This technology can be applied to membrane proteins from any host source, and, uniquely, allows purification without the protein ever being removed from a lipid bilayer. Not only do these SMA lipid particles (SMALPs) stabilise membrane proteins, allowing structural and functional studies, but they also offer opportunities to understand the local lipid environment of the host membrane. With any new or different method, questions inevitably arise about the integrity of the protein purified: does it retain its activity; its native structure; and ability to perform its function? How do membrane proteins within SMALPS perform in existing assays and lend themselves to analysis by established methods? We outline here recent work on the structure and function of membrane proteins that have been encapsulated like this in a polymer-bound lipid bilayer, and the potential for the future with this approach. This article is part of a Special Issue entitled: Beyond the Structure-Function Horizon of Membrane Proteins edited by Ute Hellmich, Rupak Doshi and Benjamin McIlwain.