detergent-free lipid phases, such as lipid vesicles or cubic phases. Such experimental systems,
however, do not lend themselves well to all experimental requirements, e.g. those of purification or
solution NMR.
A particularly daring approach, which has often failed and sometimes succeeded, is to turn the
target MP into a soluble one. This can be achieved chemically, by covalently linking to the protein
hydrophilic groups such as polyethylene glycol (making it, however, impossible to crystallize) (see
e.g. Sirokmán and Fasman 1993; Wei and Fasman 1995; Pomroy and Deber 1998). It can also be
attempted genetically, by replacing the hydrophobic residues that normally face the membrane interior
with hydrophilic ones and attempting folding in aqueous solutions. This is a difficult exercise which
has many pitfalls, including unintended interactions that lead to aggregation. One successful case is
that of the TM region of phospholamban, a homopentamer of single TM α-helices. Making the
membrane-exposed surface of the helix hydrophilic yielded water-soluble, helical pentamers, and
further design refinement gave fully folded, soluble structures (Li et al. 2001; Slovic et al. 2005a, b).
Similar efforts led to water-soluble versions of other self-associating single-TM complexes (see
DeGrado et al. 2003), as well as of a tetrameric ion channel, KcsA, each monomer of which comprises
two TM α-helices (Slovic et al. 2004; Ma et al. 2008). Most dramatically, a water-soluble version of a
GPCR, the μ-opioid receptor, was shown to fold into a stable structure that binds naltrexone with an
affinity close to that observed with the wild-type protein (Zhao et al. 2014). How general can such
approaches be and how close to the native membrane-embedded structure are those of the MPs thus
rendered water-soluble remain to be ascertained.
An alternative approach is to replace the detergent with less aggressive surfactants. Those need
not necessarily be dissociating enough to extract the target MP from its native membrane – they may
not be detergents – provided they keep MPs water-soluble after they have been extracted. This is how
APols work. Before turning to APols, however, we will go through a broad survey of other
nonconventional surfactant systems, including bicelles, nanodiscs, peptide-based systems, and
fluorinated surfactants (Chap. 3). This will set the stage for a presentation and discussion of the
original way APols work and what their advantages and drawbacks are as compared to other systems
(Chaps. 4 and 5). Chapters 6, 7, 8, 9, 10, 11, 12, 13, 14, and 15 will then be devoted to describing and
discussing the use of APols for various applications.
References
Abel, S., Dupradeau, F.Y., Raman, E.P., MacKerell, A.D., Jr., Marchi, M. (2011) Molecular simulations of
dodecyl-β-maltoside micelles in water: influence of the headgroup conformation and force field parameters.
J. Phys. Chem. B 115:487–499.
Andersen, K.K., Otzen, D.E. (2014) Folding of outer membrane protein A in the anionic biosurfactant rhamnolipid.
FEBS Lett. 588:1955–1960.
Aoudia, M., Zana, R. (1998) Aggregation behavior of sugar surfactants in aqueous solutions: Effects of temperature and
the addition of nonionic polymers. J. Colloid Interface Sci. 206:158–167.
Arachea, B.T., Sun, Z., Potente, N., Malik, R., Isailovic, D., Viola, R.E. (2012) Detergent selection for enhanced
extraction of membrane proteins. Prot. Expr. Purif. 86:12–20.
Arnold, T., Linke, D. (2008) The use of detergents to purify membrane proteins. Curr. Protoc. Protein Sci. 4:Unit
4.8.1–4.8.30.
Banerjee, P., Joo, J.B., Buse, J.T., Dawson, G. (1995) Differential solubilization of lipids along with membrane proteins
by different classes of detergents. Chem. Phys. Lipids 77:65–78.
Bhairi, S.M., Mohan, C. (2007) Detergents. A Guide to the Properties and Uses of Detergents in Biology and
Biochemistry (Calbiochem booklet). EMD Biosciences, Darmstadt, 43 p.
Bogusz, S., Venable, R.M., Pastor, R.W. (2001) Molecular dynamics simulations of octylglucoside micelles: dynamic
properties. J. Phys. Chem. B 105:8312–8321.
88
2 Extracting Membrane Proteins from Their Native Environment
however, do not lend themselves well to all experimental requirements, e.g. those of purification or
solution NMR.
A particularly daring approach, which has often failed and sometimes succeeded, is to turn the
target MP into a soluble one. This can be achieved chemically, by covalently linking to the protein
hydrophilic groups such as polyethylene glycol (making it, however, impossible to crystallize) (see
e.g. Sirokmán and Fasman 1993; Wei and Fasman 1995; Pomroy and Deber 1998). It can also be
attempted genetically, by replacing the hydrophobic residues that normally face the membrane interior
with hydrophilic ones and attempting folding in aqueous solutions. This is a difficult exercise which
has many pitfalls, including unintended interactions that lead to aggregation. One successful case is
that of the TM region of phospholamban, a homopentamer of single TM α-helices. Making the
membrane-exposed surface of the helix hydrophilic yielded water-soluble, helical pentamers, and
further design refinement gave fully folded, soluble structures (Li et al. 2001; Slovic et al. 2005a, b).
Similar efforts led to water-soluble versions of other self-associating single-TM complexes (see
DeGrado et al. 2003), as well as of a tetrameric ion channel, KcsA, each monomer of which comprises
two TM α-helices (Slovic et al. 2004; Ma et al. 2008). Most dramatically, a water-soluble version of a
GPCR, the μ-opioid receptor, was shown to fold into a stable structure that binds naltrexone with an
affinity close to that observed with the wild-type protein (Zhao et al. 2014). How general can such
approaches be and how close to the native membrane-embedded structure are those of the MPs thus
rendered water-soluble remain to be ascertained.
An alternative approach is to replace the detergent with less aggressive surfactants. Those need
not necessarily be dissociating enough to extract the target MP from its native membrane – they may
not be detergents – provided they keep MPs water-soluble after they have been extracted. This is how
APols work. Before turning to APols, however, we will go through a broad survey of other
nonconventional surfactant systems, including bicelles, nanodiscs, peptide-based systems, and
fluorinated surfactants (Chap. 3). This will set the stage for a presentation and discussion of the
original way APols work and what their advantages and drawbacks are as compared to other systems
(Chaps. 4 and 5). Chapters 6, 7, 8, 9, 10, 11, 12, 13, 14, and 15 will then be devoted to describing and
discussing the use of APols for various applications.
References
Abel, S., Dupradeau, F.Y., Raman, E.P., MacKerell, A.D., Jr., Marchi, M. (2011) Molecular simulations of
dodecyl-β-maltoside micelles in water: influence of the headgroup conformation and force field parameters.
J. Phys. Chem. B 115:487–499.
Andersen, K.K., Otzen, D.E. (2014) Folding of outer membrane protein A in the anionic biosurfactant rhamnolipid.
FEBS Lett. 588:1955–1960.
Aoudia, M., Zana, R. (1998) Aggregation behavior of sugar surfactants in aqueous solutions: Effects of temperature and
the addition of nonionic polymers. J. Colloid Interface Sci. 206:158–167.
Arachea, B.T., Sun, Z., Potente, N., Malik, R., Isailovic, D., Viola, R.E. (2012) Detergent selection for enhanced
extraction of membrane proteins. Prot. Expr. Purif. 86:12–20.
Arnold, T., Linke, D. (2008) The use of detergents to purify membrane proteins. Curr. Protoc. Protein Sci. 4:Unit
4.8.1–4.8.30.
Banerjee, P., Joo, J.B., Buse, J.T., Dawson, G. (1995) Differential solubilization of lipids along with membrane proteins
by different classes of detergents. Chem. Phys. Lipids 77:65–78.
Bhairi, S.M., Mohan, C. (2007) Detergents. A Guide to the Properties and Uses of Detergents in Biology and
Biochemistry (Calbiochem booklet). EMD Biosciences, Darmstadt, 43 p.
Bogusz, S., Venable, R.M., Pastor, R.W. (2001) Molecular dynamics simulations of octylglucoside micelles: dynamic
properties. J. Phys. Chem. B 105:8312–8321.
88
2 Extracting Membrane Proteins from Their Native Environment