6.3. Detergent-free methods – the emergence of SMALPs
Although all crystal structures of membrane proteins to date, including those synthesized in yeast, have used detergents for extraction of the protein from the lipid bilayer, the use of detergents is not without problems. As mentioned in Section 6.1, screening for conditions and detergents that effectively extract the protein yet retain structure and stability can be difficult, time consuming and expensive. The environment produced by a detergent micelle does not fully mimic the lipid bilayer environment, as not only does the bilayer provide lateral pressure to stabilize the protein structure but interactions between the protein and its annular lipids can affect protein function. Notably, the most effective detergents for extraction are often not the best detergents for crystal formation. Recently a new detergent-free method for extraction of membrane proteins has emerged using SMA co-polymers (Fig. 5). The SMA inserts into biological membranes and forms small discs of lipid bilayer (10–12 nm) surrounded by the polymer, termed SMALPs (SMA lipid particles) [9] and [10], also known as lipodisqs [98] or native nanodiscs [11]. Membrane proteins within the SMALPs retain their annular lipid bilayer environment [9], [11] and [99], yet the particles are small, stable and water soluble, allowing standard affinity chromatography methods to be used to purify a protein of interest