Formation and Properties of Membrane
Protein/Amphipol Complexes
5
Summary
Complexes between membrane proteins (MPs) and amphipols (APols) can be obtained
(i) by transferring to APols a detergent-solubilized MP, (ii) by directly extracting a
membrane-bound one, (iii) by folding in APols a denatured MP, or (iv) by synthesizing
it in vitro in the presence of APol. The complexes comprise a belt of APol that
surrounds the transmembrane region of the protein and, if available, protein-bound
lipids. Trapping with APols very generally maintains the activity of the protein, but a
few cases of reversible inhibition have been observed. Most APol-trapped MPs are
more stable, and generally much more so, than their detergent-solubilized
counterparts. Three mechanisms appear to contribute to this stabilization:
(i) reduction of the hydrophobic sink, (ii) the intrinsically less dissociating character
of APols as compared to detergents, and (iii) damping by APols of MP dynamics, which
raises the free energy barrier to unfolding. The latter phenomenon appears to be
involved in the inhibition observed with some MPs, inasmuch as it can slow down
conformational transitions. MPs trapped with APols can be transferred directly to
detergent solutions, to other APols, to lipid vesicles, to lipidic mesophases, to black
lipid films, or to cell membranes, as well as, indirectly, to nanodiscs or SMALPs.
5.1
Introduction
This present chapter deals with the way to form membrane protein/amphipol (MP/APol) complexes
and with a description of their general properties. Whereas it must be kept in mind that different MPs
may be affected in different ways by being transferred from a membrane to an APol environment and
that different APols have different properties, the detailed knowledge accumulated on the composition,
structure, and dynamics of a few particularly well-studied complexes provides a good starting point to
understanding the behavior of the others.
There are many ways to form membrane MP/APol complexes. In this chapter, we will be
principally concerned with methods that start from the folded protein, whether or not it has been
beforehand extracted from its native membrane. In Chaps. 6 and 7, we will examine two other
approaches, in which the target protein is either folded from a denatured to a functional state using
# 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_5
237
Protein/Amphipol Complexes
5
Summary
Complexes between membrane proteins (MPs) and amphipols (APols) can be obtained
(i) by transferring to APols a detergent-solubilized MP, (ii) by directly extracting a
membrane-bound one, (iii) by folding in APols a denatured MP, or (iv) by synthesizing
it in vitro in the presence of APol. The complexes comprise a belt of APol that
surrounds the transmembrane region of the protein and, if available, protein-bound
lipids. Trapping with APols very generally maintains the activity of the protein, but a
few cases of reversible inhibition have been observed. Most APol-trapped MPs are
more stable, and generally much more so, than their detergent-solubilized
counterparts. Three mechanisms appear to contribute to this stabilization:
(i) reduction of the hydrophobic sink, (ii) the intrinsically less dissociating character
of APols as compared to detergents, and (iii) damping by APols of MP dynamics, which
raises the free energy barrier to unfolding. The latter phenomenon appears to be
involved in the inhibition observed with some MPs, inasmuch as it can slow down
conformational transitions. MPs trapped with APols can be transferred directly to
detergent solutions, to other APols, to lipid vesicles, to lipidic mesophases, to black
lipid films, or to cell membranes, as well as, indirectly, to nanodiscs or SMALPs.
5.1
Introduction
This present chapter deals with the way to form membrane protein/amphipol (MP/APol) complexes
and with a description of their general properties. Whereas it must be kept in mind that different MPs
may be affected in different ways by being transferred from a membrane to an APol environment and
that different APols have different properties, the detailed knowledge accumulated on the composition,
structure, and dynamics of a few particularly well-studied complexes provides a good starting point to
understanding the behavior of the others.
There are many ways to form membrane MP/APol complexes. In this chapter, we will be
principally concerned with methods that start from the folded protein, whether or not it has been
beforehand extracted from its native membrane. In Chaps. 6 and 7, we will examine two other
approaches, in which the target protein is either folded from a denatured to a functional state using
# 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_5
237