Self-Assembly of Discoidal Phospholipid Bilayer Nanoparticles with Membrane Scaffold Proteins

Abstract

Nanoparticulate phospholipid bilayer disks were assembled from phospholipid and a class of amphipathic helical proteins termed membrane scaffold proteins (MSP). Several different MSPs were produced in high yield using a synthetic gene and a heterologous expression system and purified to homogeneity by a one-step purification. The self-assembly process begins with a mixture of the phospholipid and MSP in the presence of a detergent. Upon removal of detergent, 10-nm diameter particles form containing either saturated or unsaturated phospholipid. The ratio of components in the initial mixture was found to be crucial for formation of a monodisperse population of nanoparticles. Exploration of the phase diagram of the lamellar to phospholipid-detergent mixed micelle transition reveals that self-assembly proceeds from the mixed micellar phase. In this case a homogeneous and monodisperse population is formed. In contrast, particle formation from the detergent-phospholipid lamellar phase results in altered size, yield, composition, and heterogeneity of the resultant particles. The nanodisks contain approximately 160 saturated or 125 unsaturated lipids and can be formed from designed amphipathic α-helical scaffold proteins. The 10-nm particles can thus contain two molecules of MSP1 or a single molecule of an MSP1 fusion (MSP2). The phospholipid bilayer main phase transition temperature is preserved in the nanodisks as determined by fluorescence spectroscopy. Scanning probe microscopy shows a monolayer of nanodisks on a mica surface with a diameter of 10 nm and the thickness of a single phospholipid bilayer (5.7 nm), confirming the presence of a bilayer domain. The gentle method of self-assembly and robustness of the resulting nanodisks provides a means for generating soluble lipid bilayer membranes on the nanometer scale and opens the possibility of using these nanostructures to incorporate single membrane proteins into a native-like environment.