Integral membrane proteins (IMPs) play crucial roles in many aspects of biology by mediating the transfer of material and sig-nals between cells and their environment. It is estimated that 20–30% of all open reading frames in the human genome encode membrane proteins and that more than 50% of current pharma-ceutical agents target IMPs1. Our understanding of IMP structure and function, however, is hampered by difficulties associated with handling these proteins2. Most IMPs are not soluble in aqueous buffer because they have large hydrophobic surfaces when prop-erly folded; therefore, detergents are required to extract IMPs from the lipid bilayer and to maintain their native states in solu-tion. Mild detergents are widely used for IMP manipulation, but many membrane proteins tend to denature and/or aggregate when solubilized with these agents3, making it difficult to conduct func-tional studies, spectroscopic analysis or crystallization trials.Earlier efforts to develop amphiphiles tailored for IMP appli-cations have involved diverse strategies and achieved vary-ing levels of success. Several peptide-based designs have been explored (peptitergents4, lipopeptide detergents5, short pep-tide surfactants6) but so far have not gained broad acceptance. Amphiphilic polymers (“amphipols”7,8) and discoidal lipid bilay-ers stabilized by an amphiphilic protein scaffold (“nanodiscs”9,10) have proven to be versatile tools for studying IMPs in native-like states in aqueous solution. It is not clear, however, whether either of these approaches can yield high-quality crystals for diffraction analysis, a prominent objective of IMP studies. Furthermore, nei-ther amphipols nor nanodiscs are designed to extract IMPs from biological membranes. Recently reported agents of low molecular weight, such as hemifluorinated surfactants (HFS)8,11 and cholic acid–based amphiphiles12, have shown promising properties, but the scope of their utility remains to be explored. Thus there has been a need for amphiphiles that can extract, stabilize and pro-mote crystallization of IMPs more effectively than do current detergents. Amphiphiles with this combination of capabilities would have to be easily prepared on a large scale, which would be extremely challenging for peptide- or protein-based agents.Here we report a class of amphiphiles that show favorable behav-ior with a diverse set of membrane proteins. The design of these amphiphiles features a central quaternary carbon, which is intended to place subtle restraints on conformational flexibility13–15. Because the quaternary carbon was derived from neopentyl glycol and because the hydrophilic groups in the examples discussed here are derived from maltose, we designate these compounds maltose–neopentyl glycol (MNG) amphiphiles. The quaternary carbon distinguishes MNG architecture from conventional deter-gent structures and enables the incorporation of two hydrophilic and two lipophilic subunits. We hypothesized that the modula-tion of flexibility and distinctive orientations of hydrophilic and lipophilic surfaces would give MNG amphiphiles properties distinct from those of analogous conventional detergents. These amphiphiles are readily synthesized. We have evaluated their performance with multiple membrane proteins in diverse applica-tions, including maintenance of native IMP folding, association and function, extraction from a native membrane, growth of high-quality crystals and support of cell-free translation.
Nature Methods: “Maltose–neopentyl glycol (MNG) amphiphiles for solubilization, stabilization and crystallization of membrane proteins”
by groups from Jean-Luc Popot, Brian Kobilka, and Samuel H. Gellman labs