Effect of Polymer Composition and pH on Membrane Solubilization by Styrene-Maleic Acid Copolymers

Abstract

The styrene-maleic acid (SMA) copolymer is rapidly gaining attention as a tool in membrane research, due to its ability to directly solubilize lipid membranes into nanodisk particles without the requirement of conventional detergents. Although many variants of SMA are commercially available, so far only SMA variants with a 2:1 and 3:1 styreneto-maleic acid ratio have been used in lipid membrane studies. It is not known how SMA composition affects the solubilization behavior of SMA. Here, we systematically investigated the effect of varying the styrene/maleic acid on the properties of SMA in solution and on its interaction with membranes. Also the effect of pH was studied, because the proton concentration in the solution will affect the charge density and thereby may modulate the properties of the polymers. Using model membranes of 1,2-dimyristoyl-sn-glycero-3-phosphocholine lipids at pH > pH agg , we found that membrane solubilization is promoted by a low charge density and by a relatively high fraction of maleic acid units in the polymer. Furthermore, it was found that a collapsed conformation of the polymer is required to ensure efficient insertion into the lipid membrane and that efficient solubilization may be improved by a more homogenous distribution of the maleic acid monomer units along the polymer chain. Altogether, the results show large differences in behavior between the SMA variants tested in the various steps of solubilization. The main conclusion is that the variant with a 2:1 styrene-to-maleic acid ratio is the most efficient membrane solubilizer in a wide pH range.

Bioinspired, Size‐Tunable Self‐Assembly of Polymer–Lipid Bilayer Nanodiscs

“…One of the disadvantages of SMA polymer nanodiscs is that they tend to precipitate under low pH (<6) [16] , which could affect several biochemical protein assays. The stability of SMA-EA nanodiscs under different pH was examined by SLS experiments (See the Supporting Information Figure S5).…”

Bioinspired, Size‐Tunable Self‐Assembly of Polymer–Lipid Bilayer Nanodiscs

“…[20] One of the commonly used solid-state NMR techniques is 2D PISEMA (polarization inversion and spin exchange at magic angle) spectroscopy that has been well utilized in obtaining dynamic structure and topology of membrane proteins embedded in lipid bilayers. [21] Here,w e demonstrate the feasibility of 2D PISEMA experiment using magnetically-aligned macro-nanodiscs containing a 15 N-labeled cytochrome b 5 protein as an example.The 2D PISEMA spectrum shown in Figure 4H correlates 15 Nc hemical shifts with 1 H-15 Nd ipolar couplings.S ince cytochrome b 5 consists of ar igid transmembrane helix and am obile large soluble domain, the resonances observed in the PISEMA spectrum mostly appear from the rigid transmembrane domain while the resonances from the soluble domain of the protein are motionally averaged out. Theh elical geometry of the transmembrane helix of cytb 5 gives rise to acharacteristic "wheel"like pattern of resonances in the 2D PISEMA spectrum.…”

“…One of the disadvantages of SMA polymer nanodiscs is that they tend to precipitate under low pH (< 6), [15] which could affect several biochemical protein assays.T he stability of SMA-EA nanodiscs under different pH was examined by SLS experiments (see the Supporting Information Figure S5). Ther esults indicate that these nanodiscs are stable under acidic pH up to pH % 3.3.…”

Fast Collisional Lipid Transfer Among Polymer-Bounded Nanodiscs

“…Third, the reduced enthalpic penalty upon formation of the transition state implies only moderate disruption of lipid/lipid interactions2627. We hypothesise that the flexible nature of SMA36 enables fast reorganisation of the polymer belt upon collision of two SMALPs. This contrasts with the stiffness of the protein belt surrounding MSP nanodiscs7, which therefore exchange lipids only by much slower monomer diffusion31.…”