6.3
Amphipol-Assisted Folding of Membrane Proteins
The rationale for trying to use APols to assist the folding of MPs was that, being able to keep them in
aqueous solution under a native form more efficiently than detergents, they could provide them with a
less aggressive environment in which to seek and adopt their native structure. We shall first review
what has been achieved to date for both α-helical and β-barrel MPs (§ 6.3.1). Three short sections will
then be devoted to (i) a discussion of the possible mechanisms that underlie the success of the approach
(§ 6.3.2), (ii) challenges and prospects (§ 6.3.3), and (iii) some more general considerations about what
conclusions can be tentatively derived regarding MP folding in general (Box 6.3).
6.3.1
Which Membrane Proteins Have Been Folded in Amphipols and How
APol-assisted folding of MPs was initially demonstrated for three model MPs, an α-helical MP, BR,
and two β-barrel MPs, OmpA and FomA (Pocanschi et al. 2006). It was then extended to two more βbarrel MPs (Leney et al. 2012) and six G protein-coupled receptors (GPCRs), each of which comprises
Number of proteins
12
2
22
7
2
3
11
2
7
4
1
2
1
B
Year of first publication
0
10
20
30
40
50
60
A
α mono
α oligo
β mono β oligo
Detergent ± lipids
Vesicles, NDs
Amphipols
Others
Total
Fig. 6.4 Environments that have proven favorable to (re)folding membrane proteins in vitro. Four types
of environments have been considered: (i) detergent or mixed lipid/detergent micelles; (ii) lipid bilayers, in
the form of either lipid vesicles or nanodiscs (NDs); (iii) amphipols and other amphipathic polymers; and
(iv) other media. In A, the cumulative number of MPs that has been successfully (re)folded in each medium
is plotted as a function of time, each protein being entered once for each curve, in the year of the first
successful report. Because some MPs have been (re)folded in different media and they are counted only
once in the curve summing all folding media, the latter shows fewer proteins than the sum of the individual
curves. In B, histograms are shown of the types of media that have been used for the (re)folding of each
structural type of MP (From Popot 2014, # 2014 Elsevier Inc. All rights reserved).
338
6 Amphipol-Assisted Folding of Membrane Proteins
Amphipol-Assisted Folding of Membrane Proteins
The rationale for trying to use APols to assist the folding of MPs was that, being able to keep them in
aqueous solution under a native form more efficiently than detergents, they could provide them with a
less aggressive environment in which to seek and adopt their native structure. We shall first review
what has been achieved to date for both α-helical and β-barrel MPs (§ 6.3.1). Three short sections will
then be devoted to (i) a discussion of the possible mechanisms that underlie the success of the approach
(§ 6.3.2), (ii) challenges and prospects (§ 6.3.3), and (iii) some more general considerations about what
conclusions can be tentatively derived regarding MP folding in general (Box 6.3).
6.3.1
Which Membrane Proteins Have Been Folded in Amphipols and How
APol-assisted folding of MPs was initially demonstrated for three model MPs, an α-helical MP, BR,
and two β-barrel MPs, OmpA and FomA (Pocanschi et al. 2006). It was then extended to two more βbarrel MPs (Leney et al. 2012) and six G protein-coupled receptors (GPCRs), each of which comprises
Number of proteins
12
2
22
7
2
3
11
2
7
4
1
2
1
B
Year of first publication
0
10
20
30
40
50
60
A
α mono
α oligo
β mono β oligo
Detergent ± lipids
Vesicles, NDs
Amphipols
Others
Total
Fig. 6.4 Environments that have proven favorable to (re)folding membrane proteins in vitro. Four types
of environments have been considered: (i) detergent or mixed lipid/detergent micelles; (ii) lipid bilayers, in
the form of either lipid vesicles or nanodiscs (NDs); (iii) amphipols and other amphipathic polymers; and
(iv) other media. In A, the cumulative number of MPs that has been successfully (re)folded in each medium
is plotted as a function of time, each protein being entered once for each curve, in the year of the first
successful report. Because some MPs have been (re)folded in different media and they are counted only
once in the curve summing all folding media, the latter shows fewer proteins than the sum of the individual
curves. In B, histograms are shown of the types of media that have been used for the (re)folding of each
structural type of MP (From Popot 2014, # 2014 Elsevier Inc. All rights reserved).
338
6 Amphipol-Assisted Folding of Membrane Proteins