Amphipols and Membrane Protein
Crystallization
11
Summary
X-ray crystallography is the field of structural biology to which, to date, amphipols
have contributed the least. Complexes formed from a membrane protein (MP) and the
best characterized amphipol, A8-35, have stubbornly refused to crystallize, whereas
ternary MP/A8-35/detergent complexes yielded crystals diffracting to low resolution.
Plausible causes of these difficulties and possible ways to alleviate them will be
discussed in relation to solution measurements and other data. New tools are being
developed, including more adequate amphipols. An alternative approach relying on the
use of amphipols to deliver membrane proteins to a lipidic mesophase, where they do
crystallize, has been explored with very promising results.
11.1 Introduction
At the onset of the development of amphipols (APols), their use for membrane protein
(MP) crystallization was deemed a particularly desirable and perhaps realistic objective. The main
rationales for hope were the asset provided by the stabilization that APols afford to most MPs, the good
behavior of MP/APol complexes in solution, and the fact that – it was thought – MP/APol complexes
could be handled in surfactant-free solutions as though they were soluble proteins, in the total or neartotal absence of free surfactant. This would avoid having to expose MPs to high concentrations of
detergent, which is, as discussed in Chap. 2, a major cause of inactivation, as well as having to cope
with the phase transitions detergent solutions experience with changes of ionic strength, concentration,
or temperature.
It was of course realized that the charges carried by the first APols to be developed, A8-35 and its
congeners (Tribet et al. 1996; see Chap. 4), were far from being a favorable factor. Indeed, detergents
that carry a net charge are known not to be conducive to MP crystallization (see e.g. Privé 2007; Parker
and Newstead 2012). Work was soon initiated in two directions: (i) examining, by radiation scattering,
which salt conditions could be used to screen the electrostatic repulsion arising from these charges
(Popot et al. 2003) and (ii) developing non-ionic APols (Prata et al. 2001). Nevertheless, despite years
of efforts, not a single MP has yet been crystallized under the form of pure MP/APol complexes.
# Springer International Publishing AG, part of Springer Nature 2018
J. -L. Popot, Membrane Proteins in Aqueous Solutions, Biological and Medical Physics,
Biomedical Engineering, https://doi.org/10.1007/978-3-319-73148-3_11
497
Crystallization
11
Summary
X-ray crystallography is the field of structural biology to which, to date, amphipols
have contributed the least. Complexes formed from a membrane protein (MP) and the
best characterized amphipol, A8-35, have stubbornly refused to crystallize, whereas
ternary MP/A8-35/detergent complexes yielded crystals diffracting to low resolution.
Plausible causes of these difficulties and possible ways to alleviate them will be
discussed in relation to solution measurements and other data. New tools are being
developed, including more adequate amphipols. An alternative approach relying on the
use of amphipols to deliver membrane proteins to a lipidic mesophase, where they do
crystallize, has been explored with very promising results.
11.1 Introduction
At the onset of the development of amphipols (APols), their use for membrane protein
(MP) crystallization was deemed a particularly desirable and perhaps realistic objective. The main
rationales for hope were the asset provided by the stabilization that APols afford to most MPs, the good
behavior of MP/APol complexes in solution, and the fact that – it was thought – MP/APol complexes
could be handled in surfactant-free solutions as though they were soluble proteins, in the total or neartotal absence of free surfactant. This would avoid having to expose MPs to high concentrations of
detergent, which is, as discussed in Chap. 2, a major cause of inactivation, as well as having to cope
with the phase transitions detergent solutions experience with changes of ionic strength, concentration,
or temperature.
It was of course realized that the charges carried by the first APols to be developed, A8-35 and its
congeners (Tribet et al. 1996; see Chap. 4), were far from being a favorable factor. Indeed, detergents
that carry a net charge are known not to be conducive to MP crystallization (see e.g. Privé 2007; Parker
and Newstead 2012). Work was soon initiated in two directions: (i) examining, by radiation scattering,
which salt conditions could be used to screen the electrostatic repulsion arising from these charges
(Popot et al. 2003) and (ii) developing non-ionic APols (Prata et al. 2001). Nevertheless, despite years
of efforts, not a single MP has yet been crystallized under the form of pure MP/APol complexes.
# Springer International Publishing AG, part of Springer Nature 2018
J. -L. Popot, Membrane Proteins in Aqueous Solutions, Biological and Medical Physics,
Biomedical Engineering, https://doi.org/10.1007/978-3-319-73148-3_11
497