Abstract
A recently developed membrane mimetic system called styrene maleic acid lipid particles (SMALPs) or lipodisq nanoparticles has shown to possess significant potential for biophysical studies of membrane proteins. This new nanoparticle system is composed of lipids encircled by SMA copolymers. Previous studies showed that SMA copolymers are capable of extracting membrane proteins directly from their native environments without the assistance of detergents. However, a full structural characterization of this promising membrane mimetic system is still lacking. In this study, the formation of lipodisq nanoparticles was characterized upon addition of the membrane protein KCNE1. Initially, multi-lamellar vesicles (MLVs) containing KCNE1 (KCNE1-MLVs) at a lipid to protein molar ratio of 500/1 were prepared using a standard dialysis method. SMA copolymers were then added to KCNE1-MLVs at a series of lipid to SMA weight ratios to observe the solubilizing property of SMA in the presence of the KCNE1 membrane protein. The solubilizing process of KCNE1-MLVs by SMA copolymers undergoes a transition phase at low SMA concentrations (samples with weight ratios of 1/0.25, 1/0.5, and 1/0.75). More lipodisq nanoparticles were formed at higher SMA concentrations (Samples with weight ratios of 1/1, 1/1.25, and 1/1.5) were directly observed in the corresponding TEM images. A single sharp DLS peak was observed from the sample at the weight ratio of 1/1.5, which indicated the complete solubilization of KCNE1-MLVs. Interestingly, the critical weight ratio for empty MLVs was found to be 1/1.25 previously, which suggested that the presence of KCNE1 makes it more difficult for the solubilizing process of the SMA copolymers. Also, a TEM image of the 1/1.5 sample showed the presence of silky aggregates of excess copolymers. Overall, this study demonstrated the ability of SMA copolymers to form lipodisq nanoparticles in the presence of the membrane protein KCNE1.