RESUMEN
In the past few years there has been a growth in the use of nano-particles for stabilizing lipid membranes with embedded proteins. These bionanoparticles provide a solution to the challenging problem of membrane protein isolation by maintaining a lipid bilayer essential to protein integrity and activity. We have described the use of an amphipathic polymer (Poly(styrene-co-maleic acid); SMA) to produce discoidal nanoparticles that contain a lipid bilayer with embedded protein. However the structure of the nanoparticle itself has not yet been determined. This leaves a major gap in understanding how the SMA stabilizes the encapsulated bilayer and how the bilayer relates physically and structurally to an unecapsulated lipid bilayer. In this paper we address this issue by describing the structure of the SMA Lipid Particle (SMALP) using data from small angle neutron scattering (SANS), electron microscopy (EM), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), differential scanning calorimetry (DSC) and nuclear magnetic resonance spectroscopy (NMR). We show that the particle is disc shaped containing a polymer "bracelet" encircling the lipid bilayer. The structure and orientation of the individual components within the bilayer and polymer are determined showing that styrene moieties within SMA intercalate between the lipid acyl chains. The dimensions of the encapsulated bilayer are also determined and match those measured for a natural membrane. Taken together, the description of structure of the SMALP forms the foundation of future development and applications of SMALPs in membrane protein production and analysis.
RESUMEN
One-third of eukaryotic proteins are integrated within membranes, as are the targets of 40% of approved drugs. However, the lack of a general means of solubilizing, stabilizing and structurally characterizing active membrane proteins has frustrated efforts to understand their mechanisms and exploit their potential value. Here we report that bilayer disks formed by phospholipids and styrene maleic anhydride copolymer preserve the functional and structural integrity of alpha-helical and beta-barrel transmembrane proteins. They form 11 nm particles that are monodispersed, biocompatible, thermostable, and water-soluble, allowing diverse membrane proteins to be simply and rapidly presented for virtually any in vitro analysis.