Tandem Facial Amphiphiles for Membrane Protein Stabilization

Abstract

We describe a new type of synthetic amphiphile that is intended to support biochemical characterization of intrinsic membrane proteins. Members of this new family displayed favorable behavior with four of five membrane proteins tested, and these amphiphiles formed relatively small micelles.Membrane proteins (MPs) play crucial roles in biology, but these proteins are difficult to handle and analyze because of their physical properties. 1 The native conformations of MPs display extensive nonpolar surfaces, which is necessary for residence in a lipid bilayer but leads to denaturation and/or aggregation in an aqueous medium. Detergents, such as dodecyl-β-D-maltoside (DDM), are typically employed to render MPs soluble by coating nonpolar protein surfaces.2 However, not all MPs can be maintained in native-like conformations when solubilized with conventional detergents.3 Moreover, even when a native conformation can be achieved, the MP-detergent complex may manifest unfavorable properties with regard to structural analysis (inability to crystallize and/or too large for NMR). Since our understanding of membrane protein structure and function remains poorly developed relative to soluble proteins, there is a persistent need for new amphiphilic "assistants" that can promote solubilization and manipulation of MPs. 4Several groups have reported creative implementations of the "facial amphiphile" concept for the design of novel amphiphiles that display favorable behavior with selected membrane proteins. 5 McGregor et al., for example, have reported lipopeptides that are intended to match the width of a lipid bilayer and to form a sheath around nonpolar surfaces of MPs.5c Zhang et al. have developed cholate-based amphiphiles in which hydrophilic maltose units project from one side of the rigid and hydrophobic steroidal skeleton. 5d Here we disclose the design of "tandem facial amphiphiles" (TFAs), which contain a pair of maltosefunctionalized deoxycholate units. Unlike previous cholate-based designs, the TFAs are long enough to match bilayer width, 7 and unlike lipopeptides, the TFAs are readily synthesized in large quantities. We show that one TFA forms micelles containing only six molecules, and gellman@chem.wisc.edu; Gether@sund.ku.dk; b.byrne@imperial.ac.uk; stroud@msg.ucsf A set of four TFAs was generated from a deoxycholate-bis-maltoside building block via linkage with a diaminopropane unit ( Figure 1). Molecular mechanics calculations suggest that an extended conformation of the TFA backbone has a length that is comparable to the width of a typical lipid bilayer (~30 Å). 7 These TFAs vary in the appendage on the amide nitrogen atoms. Each amphiphile could be obtained in excellent purity (>98%) and good overall yield (~65%) in five straightforward synthetic steps with two chromatographic purifications.7 Multi-gram quantities are readily available.The TFAs displayed interesting behavior in water. TFA-0 forms a hydrogel at concentrations > 0.4 wt %, and this compound was not studied further. The other three TFAs...

Steroid-based facial amphiphiles for stabilization and crystallization of membrane proteins

“…We propose that this structural improvement over the cholate/CHAPS/CHAPSO family is key to the improved success of FAs in our current tests of MP crystallization. Of note, a related deoxycholate-based tandem FA design has been recently described for stabilization of MPs (35) and may also be useful for crystallization.…”

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

“…Such design proved to be useful to not only to stabilize membrane proteins (via tight interactions between a membrane proteins and FA owning to the large hydrophobic surface of the latter), but to also improve their crystallization (via the formation of more compact protein-detergent complexes, which can pack more tightly in a crystal, owing to the ability of FA to mediate intermolecular contacts between molecules of membrane proteins) [84]. Additionally, the longer tandem version of FA is available, where two steroid moieties are connected via a linker of varying length to match the width of a lipid bilayer [85].…”

Ice breaking in GPCR structural biology

“…McGregor et al have reported that lipopeptides (LPDs) are self-assembled into a cylindrical micelle with a width similar to that of a lipid bilayer, and further form a rigid sheath around the protein surface [55] . Zhang et al have developed cholate-based amphiphiles that project hydrophilic maltose units from one side of the steroidal skeleton, which is then further developed by the design of "tandem facial amphiphiles (TFAs)" [60,61] . The TFAs contain a pair of maltose-functionalized deoxycholate units, making it long enough to match the bilayer width.…”

“…These facial amphiphiles are very successful in reducing the micelle size. Both Zhang et al and Chae et al showed that the facial amphiphiles are only approximately 1/6 of the micelle size compared to the traditional detergents such as DDM, with an aggregation number of approximately 37 for FAs and only approximately 6 for the TFAs per PDC [60,61] . Although there is yet to be a direct application of these facial amphiphiles in Figure 3.…”

“…A brief summary of the new detergents to facilitate the GPCR structure study. (A) MNG [48] ; (B) branched detergent [47] ; (C) tandem facial amphiphiles (TFA) [61] ; (D) facial amphiphiles [60] ; (E) a cartoon about the working hypothesis of facial amphiphiles and tandem facial amphiphiles in compare with conventional detergents [60] .…”