Amphipols, Nanodiscs, and Fluorinated Surfactants: Three Nonconventional Approaches to Studying Membrane Proteins in Aqueous Solutions

Abstract

Membrane proteins (MPs) are usually handled in aqueous solutions as protein/detergent complexes. Detergents, however, tend to be inactivating. This situation has prompted the design of alternative surfactants that can be substituted for detergents once target proteins have been extracted from biological membranes and that keep them soluble in aqueous buffers while stabilizing them. The present review focuses on three such systems: Amphipols (APols) are amphipathic polymers that adsorb onto the hydrophobic transmembrane surface of MPs; nanodiscs (NDs) are small patches of lipid bilayer whose rim is stabilized by amphipathic proteins; fluorinated surfactants (FSs) resemble detergents but interfere less than detergents do with stabilizing protein/protein and protein/lipid interactions. The structure and properties of each of these three systems are described, as well as those of the complexes they form with MPs. Their respective usefulness, constraints, and prospects for functional and structural studies of MPs are discussed.

Aβ42 assembles into specific β-barrel pore-forming oligomers in membrane-mimicking environments

“…To study Aβ oligomer formation in a membrane environment, we used detergent micelles, one of the most commonly used biomimetic membrane environments to examine the structure of membrane proteins and their complexes (18,19). However, detergent micelles have been reported to compete with the factors that stabilize protein-protein interactions (20). Accordingly, we envisioned that a large excess of free micelles could act as a "hydrophobic sink," in which Aβ subunits could disperse, leading to the disruption of Aβ oligomers.…”

Involvement of distinct arrestin-1 elements in binding to different functional forms of rhodopsin

“…However, detergents sometimes disrupt the native structure of arrestins. We examined arrestin-1 structure and stability using near-UV CD in the presence of different types of micelles, bicelles, and amphipols (25). Detergents and amphipols destabilized arrestin-1, whereas three different types of bicelles yielded CD spectra very similar to those observed in buffer only (Fig.…”

An Overview of the Top Ten Detergents Used for Membrane Protein Crystallization

“…Furthermore, other alternatives exist, such as bicelles for crystallization [76]-commercial kits available (in the form of lipid-detergent mixture, typically DMPC with CHAPSO); fluorinated surfactants [46] and nanodiscs (a patch of the lipid bilayer encircled by membrane scaffolding proteins) [44] for protein stabilization. Fluorinated surfactants have not gained much attention so far, probably due to its nature-it has a hydrophobic tail with the incorporation of several fluorine atoms, rendering it lipophobic and preventing effective interactions with a membrane protein, typically causing aggregation of the latter.…”

“…For the X-ray crystallography field, it has been shown that amphipols are compatible with lipidic cubic phases [42], thus currently amphipols can only be used for in meso crystallization [43], similarly to SMALPs. There are several other approaches developed (and being actively developed) for the stabilization of membrane proteins, including nanodiscs [44], calixarens [45], fluorinated surfactants [46] and others, but they are beyond the scope of this mini-review. (From top to bottom) the concentration of detergent is increased, until the protein is extracted in the form of the complex with detergent (and some residual lipids), surrounded by free detergent-lipid micelles.…”

“…There are several other approaches developed (and being actively developed) for the stabilization of membrane proteins, including nanodiscs [44], calixarens [45], fluorinated surfactants [46] and others, but they are beyond the scope of this mini-review. …”