seven TM α-helices (Dahmane et al. 2009; Banères et al. 2011; Bazzacco et al. 2012) (Table 6.1).
We provide here a summary of the most significant results. More detailed accounts can be found in
reviews by Kleinschmidt and Popot (2014) and Le Bon et al. (2018), on which this section is based.
Because α-helical and β-barrel MPs fold according to different principles and different protocols, they
are treated separately.
Amphipol-assisted folding of membrane proteins
(# 2018 by Francis Haraux)
6.3.1.1 Amphipol-Assisted Folding of a-Helical Membrane Proteins
6.3.1.1.1 Folding of BR in A8-35
Because the recovery of its purple color makes it straightforward to observe and measure its renaturation, and it is easy to produce in large amounts, BR has served as a popular model protein in studies on
MP folding ever since the seminal work of Khorana and coworkers (Huang et al. 1981) (for reviews,
see e.g. Popot and Engelman 1990, 2000; Engelman et al. 2003; Popot 2014; Tastan et al. 2014). The
protocol followed in the first experiments aiming at folding it in APols is schematized in Fig. 6.5A. As
mentioned above (§ 6.2), when BR is solubilized in SDS, it denatures to BO, releasing its chromophore
(because the Schiff base that associates the aldehyde function of the retinal with the amine function of a
lysine residue hydrolyzes spontaneously once exposed to water), which causes an absorbance shift
from ~555 nm (dark-adapted BR) to ~382 nm (free retinal). The removal of SDS from BO/retinal in the
presence of APol A8-35 by precipitating dodecyl sulfate (DS) as potassium dodecyl sulfate (PDS) – a
protocol initially developed to refold BR into minimal amounts of lipids (Popot et al. 1987) – results in
6.3 Amphipol-Assisted Folding of Membrane Proteins
339
We provide here a summary of the most significant results. More detailed accounts can be found in
reviews by Kleinschmidt and Popot (2014) and Le Bon et al. (2018), on which this section is based.
Because α-helical and β-barrel MPs fold according to different principles and different protocols, they
are treated separately.
Amphipol-assisted folding of membrane proteins
(# 2018 by Francis Haraux)
6.3.1.1 Amphipol-Assisted Folding of a-Helical Membrane Proteins
6.3.1.1.1 Folding of BR in A8-35
Because the recovery of its purple color makes it straightforward to observe and measure its renaturation, and it is easy to produce in large amounts, BR has served as a popular model protein in studies on
MP folding ever since the seminal work of Khorana and coworkers (Huang et al. 1981) (for reviews,
see e.g. Popot and Engelman 1990, 2000; Engelman et al. 2003; Popot 2014; Tastan et al. 2014). The
protocol followed in the first experiments aiming at folding it in APols is schematized in Fig. 6.5A. As
mentioned above (§ 6.2), when BR is solubilized in SDS, it denatures to BO, releasing its chromophore
(because the Schiff base that associates the aldehyde function of the retinal with the amine function of a
lysine residue hydrolyzes spontaneously once exposed to water), which causes an absorbance shift
from ~555 nm (dark-adapted BR) to ~382 nm (free retinal). The removal of SDS from BO/retinal in the
presence of APol A8-35 by precipitating dodecyl sulfate (DS) as potassium dodecyl sulfate (PDS) – a
protocol initially developed to refold BR into minimal amounts of lipids (Popot et al. 1987) – results in
6.3 Amphipol-Assisted Folding of Membrane Proteins
339