Quantifying lipid exchage in bulk solution with protein-containing SMALP nanodiscs

Poly-Styrene-co-Maleic Acid Lipid Particles (SMALPs) are discoidal self-assembled nanoparticles containing a central lipid bilayer core stabilised by a radial amphipathic polymer belt, used to extract membrane proteins directly from cell membranes complete with annular lipids. Recent studies have shown that lipids within SMALPs rapidly exchange with other SMALPs in bulk solution as well as interfacial lipid mono/bilayers. However, this contradicts the assumption that the native lipid environment of a SMALP-encapsulated protein is maintained. To date, it is unknown what influence the presence of a protein within the SMALP has on the propensity for lipids to exchange, which is vital to understand in order to fully realise the biotechnological potential of SMALPs. We have investigated lipid exchange in protein-containing SMALPs with solid-supported bilayers using neutron reflectometry. However, this does not give information on the fate of the nanodiscs themselves. Here, we propose to use SANS with a combination of static structural measurements and time-resolved zero-contrast match point experiments to determine the rate and extent of lipid exchange between protein-containing SMALPs and large unilamellar vesicles in bulk solution, and determine the structural and compositional effect on exchange processes on the SMALPs themselves.