Fast Collisional Lipid Transfer Among Polymer-Bounded Nanodiscs

Abstract

Some styrene/maleic acid (SMA) copolymers solubilise membrane lipids and proteins to form polymer-bounded nanodiscs termed SMA/lipid particles (SMALPs). Although SMALPs preserve a lipid-bilayer core, they appear to be more dynamic than other membrane mimics. We used time-resolved Förster resonance energy transfer and small-angle neutron scattering to determine the kinetics and the mechanisms of phospholipid transfer among SMALPs. In contrast with vesicles or protein-bounded nanodiscs, SMALPs exchange lipids not only by monomer diffusion but also by fast collisional transfer. Under typical experimental conditions, lipid exchange occurs within seconds in the case of SMALPs but takes minutes to days in the other bilayer particles. The diffusional and second-order collisional exchange rate constants for SMALPs at 30 °C are kdif = 0.287 s−1 and kcol = 222 M−1s−1, respectively. Together with the fast kinetics, the observed invariability of the rate constants with probe hydrophobicity and the moderate activation enthalpy of ~70 kJ mol−1 imply that lipids exchange through a “hydrocarbon continuum” enabled by the flexible nature of the SMA belt surrounding the lipid-bilayer core. Owing to their fast lipid-exchange kinetics, SMALPs represent highly dynamic equilibrium rather than kinetically trapped membrane mimics, which has important implications for studying protein/lipid interactions in polymer-bounded nanodiscs.

Membrane Solubilization by Styrene-Maleic Acid Copolymers: Delineating the Role of Polymer Length

“…Remarkably, very fast dequenching rates were observed for nanodisks bounded by SMA 3:1-derived polymers (Fig. 6 B), in line with previous reports (18). Despite the fast lipid exchange rates observed between SMA 3:1-nanodisks, it was possible to detect slower dequenching rates for nanodisks bounded by high-M n polymers than for those bounded by low-M n polymers.…”

“…S8). This finding may serve to support the theory that lipid exchange between nanodisks shows a high Brownian dependence, thus being affected by the number of particles in solution (18).…”

Bacillus subtilis MraY in detergent-free system of nanodiscs wrapped by styrene-maleic acid copolymers

“…In any case, this implies that exchange of C35-P between detergent micelles and the lipid environment of MraY in nanodiscs is very slow, which is supported by the generally higher apparent Km of the SMA-MraY reaction. However, it was shown in a recent study using another SMA variant, that there is fast exchange of phospholipids (specifically DMPC and NBD-PE) between SMALPs [ 44 ]. This lipid exchange appeared to happen within seconds, presumably through fast collisional transfer [ 44 ].…”

Membrane biology visualized in nanometer-sized discs formed by styrene maleic acid polymers

“…The lipid bilayer structure appears to be less affected by polymers containing a lower amount of styrene to maleic acid, with a 2:1 ratio being less disruptive than a 3:1 ratio [11]. The exchange of lipids between particles is relatively rapid compared to other nanodisc systems, and is mediated by diffusion of monomeric lipid molecules out of discs and, particularly at high concentrations, by collisions between discs [12]. The dynamic nature of the SMA presumably allows lipids to be introduced or removed from a SMALP relatively easily, and also allows proteins to be transferred back into a liposome or other bilayer system [13].…”