Abstract
Styrene/maleic acid (SMA) and related copolymers are attracting great interest because they solubilise membrane proteins and lipids to form polymer-encapsulated nanodiscs. These nanodiscs retain a lipid-bilayer core surrounded by a polymer rim and can harbour a membrane protein or a membrane-protein complex. SMA exists in different styrene/maleic acid molar ratios, which results in differences in hydrophobicity and solubilisation properties. We have recently demonstrated fast collisional lipid transfer among nanodiscs encapsulated by the relatively hydrophobic copolymer SMA(3:1). Here, we used time-resolved Förster resonance energy transfer to quantify the lipid-transfer kinetics among nanodiscs bounded by SMA(2:1), a less hydrophobic copolymer that is superior in terms of lipid and membrane-protein solubilisation. Moreover, we assessed the effects of ionic strength and, thereby, the role of Coulombic repulsion in the exchange of lipid molecules among these polyanionic nanodiscs. Collisional lipid transfer was slower among SMA(2:1) nanodiscs (kcol = 5.9 M−1 s−1) than among SMA(3:1) nanodiscs (kcol = 222 M−1 s−1) but still two to three orders of magnitude faster than diffusional lipid exchange among protein-encapsulated nanodiscs or vesicles. Increasing ionic strength accelerated lipid transfer in a manner predicted by the Davies equation, an empirical extension of the Debye–Hückel limiting law, or an extended equation taking into account the finite size of the nanodiscs. Using the latter approach, quantitative agreement between experiment and theory was achieved for an effective nanodisc charge number of z ≈ –33, which is an order of magnitude less than their nominal overall charge.
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Acknowledgements
Bartholomäus Danielczak (TUK) is gratefully thanked for helping with TR-FRET experiments. We thank Dr. Rodrigo Cuevas Arenas, Bartholomäus Danielczak, Erik Frotscher, Johannes Klingler, Florian Mahler, and Dr. Carolyn Vargas (all TUK) for fruitful discussions. This work was supported by the Carl Zeiss Foundation through the Centre for Lipidomics (CZSLip) and the Deutsche Forschungsgemeinschaft (DFG) through International Research Training Group (IRTG) 1830.
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Grethen, A., Glueck, D. & Keller, S. Role of Coulombic Repulsion in Collisional Lipid Transfer Among SMA(2:1)-Bounded Nanodiscs. J Membrane Biol 251, 443–451 (2018). https://doi.org/10.1007/s00232-018-0024-0
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DOI: https://doi.org/10.1007/s00232-018-0024-0