Characterization of divalent cation interactions with AASTY native nanodiscs

Abstract

Amphiphilic copolymers show great promise in extracting membrane proteins directly from lipid bilayers into 'native nanodiscs'. However, many such copolymers are polyanionic and sensitive to divalent cations, limiting their applicability towards Ca2+ or Mg2+ dependent proteins. Here, we characterize the Ca2+ and Mg2+ sensitivity of poly(acrylic acid-co-styrene) (AASTY) copolymers using analytical UV and fluorescent size exclusion chromatography, enabling us to separate signals from nanodiscs, copolymers, and soluble aggregates. Determination of free Ca2+ ion concentrations in the presence of copolymer shows that divalent cation tolerance is dependent on not only specific characteristics of a copolymer, but also on its concentration. We see that high ionic strength protects against aggregation facilitated by divalent cations, which is prominent in nanodiscs isolated from excess free copolymer through dialysis. Overall, we conclude that the behavior of amphiphilic copolymers in the presence of divalent cations is more complex than precipitation beyond a specific cation concentration.