Final Comments
In the course of this book, we have examined and discussed the various methods by which a membrane
protein can be kept water-soluble after being extracted from its native environment. Particular
emphasis has been put on amphipols, with the design and development of which my colleagues and
I have been most closely associated. At the time of concluding, one must ask what, after more than
20 years of development, is the current situation of amphipols in the general landscape of membrane
protein biochemistry and biophysics. It is fair to say that it is both rewarding and frustrating.
It is rewarding on two main counts. First, the use of amphipols has been validated for a vast
variety of membrane proteins and hosts of applications. Their general properties and those of
membrane protein/amphipol complexes have been studied in detail with a variety of amphipols and
proteins, providing the novice experimenter with a vast body of data to peer upon in order to decide
which amphipol to use, how best to proceed, what to expect, and what to be wary of. At the date of this
writing, close to 20 distinct amphipols have been synthesized, tested, and validated for biochemical
use, to a more or less detailed extent, and close to 100 distinct membrane proteins have been trapped in
amphipols and, when assayed, shown to be in their native state and, if testable, functional. A score of
labeled or functionalized derivatives adapted to an amazing variety of uses have been synthesized and
validated. The toolbox has thus become increasingly diversified. In several fields, when launching a
new experimental study, testing amphipols along with other surfactants is becoming increasingly
routine.
Second, amphipols have started to yield biological dividends. To cite only two examples, they
have been used to fold hard-to-express membrane proteins, such as G protein-coupled receptors, and
make those available for biochemical and biophysical studies, either in amphipols or following transfer
to other environments. Amphipols have become routine tools in electron microscopy, where some half
of the high-resolution single-particle cryo-EM structures of MPs that are currently published in a
steady stream resort to them. More scattered contributions have also appeared in the fields of NMR,
radiation scattering, mass spectrometry, membrane protein folding, synthesis or immobilization,
biomedical applications, etc. Perspectives for the development of amphipols and their applications
are extremely rich. I have attempted to discuss them at the end of each technical chapter. These
suggestions and speculations will not be repeated here, but it should be stressed that almost any kind of
labeling or tagging is accessible to chemical synthesis, and that whenever the physical-chemical or
biochemical properties of the reference amphipol, A8-35, make it unsuitable for a given application,
alternative amphipols can be resorted to. Tagged or labeled versions of these alternative amphipols
have also started to appear.
# 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
683
In the course of this book, we have examined and discussed the various methods by which a membrane
protein can be kept water-soluble after being extracted from its native environment. Particular
emphasis has been put on amphipols, with the design and development of which my colleagues and
I have been most closely associated. At the time of concluding, one must ask what, after more than
20 years of development, is the current situation of amphipols in the general landscape of membrane
protein biochemistry and biophysics. It is fair to say that it is both rewarding and frustrating.
It is rewarding on two main counts. First, the use of amphipols has been validated for a vast
variety of membrane proteins and hosts of applications. Their general properties and those of
membrane protein/amphipol complexes have been studied in detail with a variety of amphipols and
proteins, providing the novice experimenter with a vast body of data to peer upon in order to decide
which amphipol to use, how best to proceed, what to expect, and what to be wary of. At the date of this
writing, close to 20 distinct amphipols have been synthesized, tested, and validated for biochemical
use, to a more or less detailed extent, and close to 100 distinct membrane proteins have been trapped in
amphipols and, when assayed, shown to be in their native state and, if testable, functional. A score of
labeled or functionalized derivatives adapted to an amazing variety of uses have been synthesized and
validated. The toolbox has thus become increasingly diversified. In several fields, when launching a
new experimental study, testing amphipols along with other surfactants is becoming increasingly
routine.
Second, amphipols have started to yield biological dividends. To cite only two examples, they
have been used to fold hard-to-express membrane proteins, such as G protein-coupled receptors, and
make those available for biochemical and biophysical studies, either in amphipols or following transfer
to other environments. Amphipols have become routine tools in electron microscopy, where some half
of the high-resolution single-particle cryo-EM structures of MPs that are currently published in a
steady stream resort to them. More scattered contributions have also appeared in the fields of NMR,
radiation scattering, mass spectrometry, membrane protein folding, synthesis or immobilization,
biomedical applications, etc. Perspectives for the development of amphipols and their applications
are extremely rich. I have attempted to discuss them at the end of each technical chapter. These
suggestions and speculations will not be repeated here, but it should be stressed that almost any kind of
labeling or tagging is accessible to chemical synthesis, and that whenever the physical-chemical or
biochemical properties of the reference amphipol, A8-35, make it unsuitable for a given application,
alternative amphipols can be resorted to. Tagged or labeled versions of these alternative amphipols
have also started to appear.
# 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
683