Nanodisc Membrane Proteins

Transmembrane proteins mediate a broad range of critical cellular processes, including signal transduction, cell-to-cell interaction, and the transport of substances into and out of cells. For this reason, transmembrane proteins represent important drug targets. However, because transmembrane proteins mimic the amphipathic properties of the lipid bilayer, isolating them in their native form can be challenging. Once transmembrane proteins are solubilized in a standard aqueous buffer, the hydrophobic regions clump together, altering both protein structure and function. Although the addition of a solubilizing agent (i.e., a detergent) can mitigate clumping, this approach prohibits the use of transmembrane proteins for certain applications, especially those involving live cells, since the solubilizing agent can irreversibly damage cell membranes. Nanodiscs, first described by Sligar et al. in 2002, were developed to enable the removal of transmembrane proteins from cell membranes while retaining their original physiological properties. These small, disc-shaped structures (usually around 10-50 nm in diameter, depending on the size of the transmembrane protein that they contain) consist of a phospholipid bilayer that is held together by a stabilizing belt. Nanodiscs are divided into two main categories based on their composition. Membrane scaffold protein (MSP) nanodiscs consist of artificial lipids such as dimyristoyl-glycero-phosphocholine (DMPC), palmitoyl-oleoyl-phosphatidylcholine (POPC), or phosphatidylglycerol (DMPG), surrounded by MSPs that are typically derived from apolipoprotein A1 (Apo A1), the primary protein component of human high-density lipoprotein (HDL) particles. In contrast, synthetic nanodiscs consist of native cell phospholipids surrounded by a synthetic polymer such as di-isobutylene-maleic acid (DIBMA), styrene maleic anhydride (SMA), or poly (acrylic acid-co-styrene) (AASTY). A main advantage of synthetic nanodiscs is that the polymer acts as both a solubilizer and a stabilizer, meaning that no detergent is required. Also, because synthetic nanodiscs use the native cell membrane as the phospholipid source, only transmembrane proteins from living cells are isolated. Synthetic nanodisc transmembrane proteins have utility for a broad range of applications. These include ELISA, surface plasmon resonance (SPR) affinity analysis, and various cell-based assays, as well as chimeric antigen receptor (CAR) expression analysis, protein crystallization studies, and phage display screening. In addition, synthetic nanodisc transmembrane proteins are useful tools for antibody production and have potential to be used as vaccines.