APols (Chap. 6) or synthesized in vitro with APols as the accepting medium (Chap. 7). The next two
chapters will deal with the optical properties of MP/APol complexes (Chap. 8) and other solution
studies (Chap. 9). Excerpts of these and other data to be presented in other chapters, particularly NMR
and electron microscopy (EM) data (Chaps. 10 and 12, respectively), will be used in the present one to
draw an integrated picture of MP/APol complexes.
In this chapter, we will also consider three further important aspects of working with APols:
(i) The effects of APols on the stability of MPs and the mechanisms underlying these effects;
(ii) Their influence on the dynamics and functional properties of MPs;
(iii) Transferring APol-trapped MPs from APols to other surfactants, such as other APols,
detergents, nanodiscs, styrene maleic acid lipid particles (SMALPs), or lipid vesicles.
5.2
Forming Membrane Protein/Amphipol Complexes
At the time of this writing, close to 100 distinct integral MPs have been trapped with one or the other
APol (most of which are compiled in Table 5.1), a number that has kept increasing steadily over the
past 20 years (Fig. 5.1). With some 150 references, Table 5.1 looks quite formidable. However, it has
been laid out so that the reader interested in finding out what has been published on which MP or type
of MP using which APol can locate it at a glance. Such a search is extremely time-consuming, if not
impossible, to do via databanks, as most articles do not include “amphipol” among their keywords,
much less the specific APol that has been used. As regards applying specific techniques to studying
APol-trapped MPs, some examples are given in Table 5.4, and exhaustive lists will be provided in the
chapters dealing with each technique.
MPs that have been trapped in APols cover all types of functions and structures, monomeric or
oligomeric, folded into α-helix bundles or into β-barrels (summarized in Table 5.2), and their masses
range from ~3 kDa (a single transmembrane (TM) α-helix; see e.g. Stangl et al. 2014) to more than
1 MDa (e.g. the mitochondrial I 1 III 2 IV 1 supercomplex; Althoff et al. 2011) (Table 5.1, Column 5).
This diversity provides a couple of interesting indications about the generality of the use of APols: first,
they can trap and keep in their native state rugged MPs – most β-barrel ones – as well as fragile ones:
most α-helical MPs, including the particularly delicate G protein-coupled receptors (GPCRs); second,
at variance with nanodiscs (NDs), there does not seem to be an upper limit as to the dimensions of the
TM regions APols can encompass and keep water-soluble. One should note, however, that whereas all
types of TM proteins have been trapped and kept water-soluble using APols (Table 5.2), no report has
been published yet about using them with monotopic MPs. There does not seem to be any reason why
this should be problematic. Indeed, bacterial sulfide-quinone reductase expressed in E. coli C41(DE3)
and purified by immobilized-metal affinity chromatography (IMAC) in the presence of 0.03%
n-dodecyl-β-D-maltopyranoside (DDM) has been successfully transferred to A8-35, the detergent
being removed using Bio-Beads. Sulfide-oxidizing activity was observed, albeit with a reduced V max
(A.H. Abbas, F. Bouillaud and colleagues, unpublished observations).
Experiments in which APols have been used to stabilize other nanomaterials, such as quantum
dots (Luccardini et al. 2006; Qi and Gao 2008; Sebai et al. 2012; Booth et al. 2013; Lim et al. 2015) or
siRNA (Li et al. 2015b), are beyond the frame of this book and will not be reviewed here.
Given the many different APols that have been tested, the number of distinct MP/APol
complexes that have been described reached >130 by the spring of 2017. Their distribution between
the various structural types of MPs and of APols is summarized in Table 5.2.
238
5 Formation and Properties of Membrane Protein/Amphipol Complexes
chapters will deal with the optical properties of MP/APol complexes (Chap. 8) and other solution
studies (Chap. 9). Excerpts of these and other data to be presented in other chapters, particularly NMR
and electron microscopy (EM) data (Chaps. 10 and 12, respectively), will be used in the present one to
draw an integrated picture of MP/APol complexes.
In this chapter, we will also consider three further important aspects of working with APols:
(i) The effects of APols on the stability of MPs and the mechanisms underlying these effects;
(ii) Their influence on the dynamics and functional properties of MPs;
(iii) Transferring APol-trapped MPs from APols to other surfactants, such as other APols,
detergents, nanodiscs, styrene maleic acid lipid particles (SMALPs), or lipid vesicles.
5.2
Forming Membrane Protein/Amphipol Complexes
At the time of this writing, close to 100 distinct integral MPs have been trapped with one or the other
APol (most of which are compiled in Table 5.1), a number that has kept increasing steadily over the
past 20 years (Fig. 5.1). With some 150 references, Table 5.1 looks quite formidable. However, it has
been laid out so that the reader interested in finding out what has been published on which MP or type
of MP using which APol can locate it at a glance. Such a search is extremely time-consuming, if not
impossible, to do via databanks, as most articles do not include “amphipol” among their keywords,
much less the specific APol that has been used. As regards applying specific techniques to studying
APol-trapped MPs, some examples are given in Table 5.4, and exhaustive lists will be provided in the
chapters dealing with each technique.
MPs that have been trapped in APols cover all types of functions and structures, monomeric or
oligomeric, folded into α-helix bundles or into β-barrels (summarized in Table 5.2), and their masses
range from ~3 kDa (a single transmembrane (TM) α-helix; see e.g. Stangl et al. 2014) to more than
1 MDa (e.g. the mitochondrial I 1 III 2 IV 1 supercomplex; Althoff et al. 2011) (Table 5.1, Column 5).
This diversity provides a couple of interesting indications about the generality of the use of APols: first,
they can trap and keep in their native state rugged MPs – most β-barrel ones – as well as fragile ones:
most α-helical MPs, including the particularly delicate G protein-coupled receptors (GPCRs); second,
at variance with nanodiscs (NDs), there does not seem to be an upper limit as to the dimensions of the
TM regions APols can encompass and keep water-soluble. One should note, however, that whereas all
types of TM proteins have been trapped and kept water-soluble using APols (Table 5.2), no report has
been published yet about using them with monotopic MPs. There does not seem to be any reason why
this should be problematic. Indeed, bacterial sulfide-quinone reductase expressed in E. coli C41(DE3)
and purified by immobilized-metal affinity chromatography (IMAC) in the presence of 0.03%
n-dodecyl-β-D-maltopyranoside (DDM) has been successfully transferred to A8-35, the detergent
being removed using Bio-Beads. Sulfide-oxidizing activity was observed, albeit with a reduced V max
(A.H. Abbas, F. Bouillaud and colleagues, unpublished observations).
Experiments in which APols have been used to stabilize other nanomaterials, such as quantum
dots (Luccardini et al. 2006; Qi and Gao 2008; Sebai et al. 2012; Booth et al. 2013; Lim et al. 2015) or
siRNA (Li et al. 2015b), are beyond the frame of this book and will not be reviewed here.
Given the many different APols that have been tested, the number of distinct MP/APol
complexes that have been described reached >130 by the spring of 2017. Their distribution between
the various structural types of MPs and of APols is summarized in Table 5.2.
238
5 Formation and Properties of Membrane Protein/Amphipol Complexes