3.6
Nonconventional Surfactants Used for Handling Membrane Proteins
in Aqueous Solutions: An Overview
At this point in the book, the reader could easily be forgiven for being somewhat bewildered: so many
solutions offered for what seems essentially (but is not really) a single problem, handling MPs in
aqueous solution under a stable form? We will come back to this issue in the final chapter on
Conclusion and Perspectives. However, before embarking onto the discussion of what APols are
and what they are good for, an orienting map and some guidelines could perhaps be useful.
A first point to keep in mind is that there is no panacea: not a single system will, at the current
point in time (and, according to all probability, in the future), provide optimal conditions for any kind
of experiment to be performed on any MP. To take a couple of obvious examples, stability is essential
in some experiments, whereas the formation of small complexes is a prerequisite for others. Stability
over days is a must for drug-screening experiments, for crystallization in solution, and, to a lesser
extent, for NMR measurements and neutron scattering measurements, which typically require many
hours of data collection. The formation of small complexes is an absolute requirement for solution
NMR measurements, and the presence of as little bound surfactant as possible is essential for most
solution crystallization experiments. Other experiments, such as most optical spectroscopy ones, or
functional measurements, are generally not dependent on the protein being trapped in small complexes
and can usually be completed in hours. The priority then shifts from long-term stability to the absence
of interference with the measurements and/or as little perturbation as possible of the protein’s
environment.
Respecting as much as is possible the interactions that a MP establishes in situ with its neighbors
is highly desirable. Some experiments, such as optical, EPR, and some NMR spectroscopy measurements, can be carried in vivo, which is, to an extent, ideal. However, for most structural explorations,
and for many functional ones, the protein must be separated from most of the rest of the cell. Preserving
an absolutely unchanged environment is something that cannot be done, even when working with
“intact” membrane fragments: the lipid and protein composition and their asymmetry may be
maintained, but not that of the medium on each side of the membrane – ion and small molecule
composition, electrostatic and redox potential, interactions with intra- and extracellular solutes and
matrices, and so forth. When extracting MPs from the membrane becomes unavoidable, as for
purification, the problems worsen: some protein/protein interactions may have to be pried out, while
others must be maintained. The bulk of the lipids has to go, whereas it is often essential that at least
some of the protein/lipid interactions be preserved, or recreated.
Figure 3.26 presents a rough, somewhat subjective map of the way the various systems we have
considered hitherto, as well as the APols we will discuss next, distribute as a function of two criteria:
how aggressive the surfactant or surfactant mixture used is (“detergency”) and how similar to the
native environment is that experienced by the solubilized MP. Detergency is understood as the ability
of the surfactant or surfactant mixture to separate membrane components one from another and,
therefore, to extract MPs from biological membranes and to compete with natural protein/protein
and protein/lipid interactions, a key factor in destabilizing MPs (see Chap. 2, § 2.4). Whether the
environment resembles the natural one does not depend only on the presence of a bilayer but also, for
less membrane-like systems, on the ability of the protein to retain or rebind lipids. Pure detergents used
in the absence of lipids thus provide less native-like an environment than fluorinated surfactants or
APols, whose structure is highly non-biological but which favor the retention of lipids.
Starting from the upper right corner of the map, liposomes – not a soluble system and, therefore,
not discussed in this book – are arguably the most natural-looking system and one of the least
aggressive. However, they cannot faithfully reproduce the asymmetry of lipid composition and
3.6 Nonconventional Surfactants Used for Handling Membrane Proteins in Aqueous. . .
133
Nonconventional Surfactants Used for Handling Membrane Proteins
in Aqueous Solutions: An Overview
At this point in the book, the reader could easily be forgiven for being somewhat bewildered: so many
solutions offered for what seems essentially (but is not really) a single problem, handling MPs in
aqueous solution under a stable form? We will come back to this issue in the final chapter on
Conclusion and Perspectives. However, before embarking onto the discussion of what APols are
and what they are good for, an orienting map and some guidelines could perhaps be useful.
A first point to keep in mind is that there is no panacea: not a single system will, at the current
point in time (and, according to all probability, in the future), provide optimal conditions for any kind
of experiment to be performed on any MP. To take a couple of obvious examples, stability is essential
in some experiments, whereas the formation of small complexes is a prerequisite for others. Stability
over days is a must for drug-screening experiments, for crystallization in solution, and, to a lesser
extent, for NMR measurements and neutron scattering measurements, which typically require many
hours of data collection. The formation of small complexes is an absolute requirement for solution
NMR measurements, and the presence of as little bound surfactant as possible is essential for most
solution crystallization experiments. Other experiments, such as most optical spectroscopy ones, or
functional measurements, are generally not dependent on the protein being trapped in small complexes
and can usually be completed in hours. The priority then shifts from long-term stability to the absence
of interference with the measurements and/or as little perturbation as possible of the protein’s
environment.
Respecting as much as is possible the interactions that a MP establishes in situ with its neighbors
is highly desirable. Some experiments, such as optical, EPR, and some NMR spectroscopy measurements, can be carried in vivo, which is, to an extent, ideal. However, for most structural explorations,
and for many functional ones, the protein must be separated from most of the rest of the cell. Preserving
an absolutely unchanged environment is something that cannot be done, even when working with
“intact” membrane fragments: the lipid and protein composition and their asymmetry may be
maintained, but not that of the medium on each side of the membrane – ion and small molecule
composition, electrostatic and redox potential, interactions with intra- and extracellular solutes and
matrices, and so forth. When extracting MPs from the membrane becomes unavoidable, as for
purification, the problems worsen: some protein/protein interactions may have to be pried out, while
others must be maintained. The bulk of the lipids has to go, whereas it is often essential that at least
some of the protein/lipid interactions be preserved, or recreated.
Figure 3.26 presents a rough, somewhat subjective map of the way the various systems we have
considered hitherto, as well as the APols we will discuss next, distribute as a function of two criteria:
how aggressive the surfactant or surfactant mixture used is (“detergency”) and how similar to the
native environment is that experienced by the solubilized MP. Detergency is understood as the ability
of the surfactant or surfactant mixture to separate membrane components one from another and,
therefore, to extract MPs from biological membranes and to compete with natural protein/protein
and protein/lipid interactions, a key factor in destabilizing MPs (see Chap. 2, § 2.4). Whether the
environment resembles the natural one does not depend only on the presence of a bilayer but also, for
less membrane-like systems, on the ability of the protein to retain or rebind lipids. Pure detergents used
in the absence of lipids thus provide less native-like an environment than fluorinated surfactants or
APols, whose structure is highly non-biological but which favor the retention of lipids.
Starting from the upper right corner of the map, liposomes – not a soluble system and, therefore,
not discussed in this book – are arguably the most natural-looking system and one of the least
aggressive. However, they cannot faithfully reproduce the asymmetry of lipid composition and
3.6 Nonconventional Surfactants Used for Handling Membrane Proteins in Aqueous. . .
133