Formation of pH-Resistant Monodispersed Polymer-Lipid Nanodiscs

Polymer lipid nanodiscs are an invaluable system for structural and functional studies of membrane proteins in their near-native environment. Despite the recent advances in the development and usage of polymer lipid nanodisc systems, lack of control over size and poor tolerance to pH and divalent metal ions are major limitations for further applications. A facile modification of a low-molecular-weight styrene maleic acid copolymer is demonstrated to form monodispersed lipid bilayer nanodiscs that show ultra-stability towards divalent metal ion concentration over a pH range of 2.5 to 10. The macro-nanodiscs (>20 nm diameter) show magnetic alignment properties that can be exploited for high-resolution structural studies of membrane proteins and amyloid proteins using solid-state NMR techniques. The new polymer, SMA-QA, nanodisc is a robust membrane mimetic tool that offers significant advantages over currently reported nanodisc systems.

Spontaneous Lipid Nanodisc Fomation by Amphiphilic Polymethacrylate Copolymers

There is a growing interest in the use of lipid bilayer nanodiscs for various biochemical and biomedical applications. Among the different types of nanodiscs, the unique features of synthetic polymer-based nanodiscs have attracted additional interest. A styrene-maleic acid (SMA) copolymer demonstrated to form lipid nanodiscs has been used for structural biology related studies on membrane proteins. However, the application of SMA polymer based lipid nanodiscs is limited because of the strong absorption of the aromatic group interfering with various experimental measurements. Thus, there is considerable interest in the development of other molecular frameworks for the formation of polymer-based lipid nanodiscs. In this study, we report the first synthesis and characterization of a library of polymethacrylate random copolymers as alternatives to SMA polymer. In addition, we experimentally demonstrate the ability of these polymers to form lipid bilayer nanodiscs through the fragmentation of lipid vesicles by means of light scattering, electron microscopy, differential scanning calorimetry, and solution and solid-state NMR experiments. We further demonstrate a unique application of the newly developed polymer for kinetics and structural characterization of the aggregation of human islet amyloid polypeptide (also known as amylin) within the lipid bilayer of the polymer nanodiscs using thioflavin-T-based fluorescence and circular dichroism experiments. Our results demonstrate that the reported new styrene-free polymers can be used in high-throughput biophysical experiments. Therefore, we expect that the new polymer nanodiscs will be valuable in the structural studies of amyloid proteins and membrane proteins by various biophysical techniques.

pH Tunable and Divalent Metal Ion Tolerant Polymer Lipid Nanodiscs

The development and applications of detergent-free membrane mimetics have been the focus for the high-resolution structural and functional studies on membrane proteins. The introduction of lipid nanodiscs has attracted new attention toward the structural biology of membrane proteins and also enabled biomedical applications. Lipid nanodiscs provide a native lipid bilayer environment similar to the cell membrane surrounded by a belt made up of proteins or peptides. Recent studies have shown that the hydrolyzed form of styrene maleic anhydride copolymer (SMA) has the ability to form lipid nanodiscs and has several advantages over protein or peptide based nanodiscs. SMA polymer lipid nanodiscs have become very important for structural biology and nanobiotechnological applications. However, applications of the presently available polymer nanodiscs are limited by their instability toward divalent metal ions and acidic conditions. To overcome the limitations of SMA nanodiscs and to broaden the potential applications of polymer nanodiscs, the present study investigates the tunability of SMA polymer nanodiscs by systematically modifying the maleic acid functional group. The two newly developed polymers and subsequent lipid nanodiscs were characterized using solid-state NMR, FT-IR, TEM, and DLS experiments. The pH dependence and metal ion stability of these nanodiscs were studied using static light scattering and FTIR. The reported polymer nanodiscs exhibit unique pH dependent stability based on the modified functional group and show a high tolerance toward divalent metal ions. We also show these tunable nanodiscs can be used to encapsulate and stabilize a polyphenolic natural product curcumin.

Bioinspired, Size-Tunable Self-Assembly of Polymer-Lipid Bilayer Nanodiscs

Polymer-based nanodiscs are valuable tools in biomedical research that can offer a detergent-free solubilization of membrane proteins maintaining their native lipid environment. Herein, we introduce a novel ca. 1.6 kDa SMA-based polymer with styrene:maleic acid moieties that can form nanodiscs containing a planar lipid bilayer which are useful to reconstitute membrane proteins for structural and functional studies. The physicochemical properties and the mechanism of formation of polymer-based nanodiscs are characterized by light scattering, NMR, FT-IR, and TEM. A remarkable feature is that nanodiscs of different sizes, from nanometer to sub-micrometer diameter, can be produced by varying the lipid-to-polymer ratio. The small-size nanodiscs (up to ca. 30 nm diameter) can be used for solution NMR spectroscopy studies whereas the magnetic-alignment of macro-nanodiscs (diameter of > ca. 40 nm) can be exploited for solid-state NMR studies on membrane proteins.