Bicelles have been extensively used for NMR studies of molecules that interact with membranes,
such as drugs, peptides that adsorb onto or insert into them, or very simple, typically dimeric bitopic
MP fragments (see e.g. Bocharov et al. 2007, 2010), as well as that of soluble molecules that orient
in the presence of oriented micelles (reviewed by Prosser et al. 2006 and Dürr et al. 2013). Of the ~140
NMR structures of TM proteins or peptides listed on www.drorlist.com/nmr/MPNMR.html, the site
maintained by Dror E. Warschawski, the determination of only a dozen has resorted to bicelles, often
in combination with other media (see Warschawski et al. 2011). The molecular mass of the largest of
those (cytochrome b 5 ) does not exceed 13 kDa. To the outside observer, a rapid survey of the
combined use of bicelles and NMR to study MP structure leaves the impression that establishing de
novo a structure using only data collected with bicelles is quite difficult and not the strongest point of
the approach. Indeed, despite 25 years of work on bicelles, all solution-NMR MP structures whose
monomers comprise more than a couple of TM segments have been obtained in detergent solutions
(see www.drorlist.com/nmr/MPNMR.html; reviewed by Marcotte and Auger 2005; Poget and Girvin
2007; Kim et al. 2009; Nietlispach and Gautier 2011; Warschawski et al. 2011). It is probably telling
that the first NMR structure of a G protein-coupled receptor (GPCR) was derived from solid-state
NMR data collected on liposome-reconstituted preparations rotated at the magic angle (Park et al.
2012), even though the group that carried out this study, that of Stanley J. Opella, has been a major
contributor to the development of bicelle technology (see e.g. Park et al. 2006, 2011a, c).
Bicelle-based NMR seems of greater use in a host of situations where it permits to complement
and extend data collected either by solution and/or solid-state NMR in other media, crystallography, or
electron microscopy (EM). Because bicelles comprise a bilayer region, in which MPs preferentially
reside (cf. Lee et al. 2008), they can be used to examine whether the protein structure is or not affected
by a purely detergent environment, a procedure often resorted to (see e.g. Fanucci et al. 2003; Howell
et al. 2005; Poget et al. 2007; Poget and Girvin 2007; Lau et al. 2008; Gautier et al. 2010), and to
determine the tilt of α-helices or β-barrels with respect to the bilayer plane (see e.g. Lindberg et al.
2003; De Angelis et al. 2004; Howell et al. 2005; Triba et al. 2006; Mahalakshmi and Marassi 2008),
an information that is generally missing in X-ray and EM structures and always absent from solutionNMR ones. Bicelles are also useful for examining conformational transitions that may not be easily
accessible to crystallography and may be perturbed by detergents (see e.g. Morrison et al. 2011;
Gustavsson et al. 2012) or for studying the consequences of the binding of drugs to MPs in a more
physiological environment than a detergent belt can provide (see e.g. Cui et al. 2010; Park et al.
2011b). They also provide an opportunity to study MP/lipid interactions and their effects on MP
function, structure, and oligomerization under much better conditions than in detergent solutions.
Furthermore, the fact that they permit to study the same MP in a continuous series of environments that
range from pure detergent solution to isotropic bicelles, both of them accessible to solution NMR, and
to aligned large bicelles and sheets or vesicles, which are a medium of choice for solid-state NMR
(Fig. 3.3), offers a unique opportunity to check and complete the information gathered using one
approach by that collected using another. This is an edge that bicelles have over nanodiscs, which, as
will be discussed in § 3.3, present a very interesting alternative when it comes to studying MPs in a
lipid bilayer environment by solution NMR but whose size can be adjusted only between rather narrow
limits.
3.2.2
Bicelles and Membrane Protein Crystallography
It is somewhat ironic that, while it is their use for NMR studies that provided the primary impetus to the
development of bicelles as hosts for MPs, the most important contribution bicelles have made to
structural studies of polytopic MPs is in the field of crystallography. Indeed, in the early 1990s, Salem
3.2 Bicelles
103
such as drugs, peptides that adsorb onto or insert into them, or very simple, typically dimeric bitopic
MP fragments (see e.g. Bocharov et al. 2007, 2010), as well as that of soluble molecules that orient
in the presence of oriented micelles (reviewed by Prosser et al. 2006 and Dürr et al. 2013). Of the ~140
NMR structures of TM proteins or peptides listed on www.drorlist.com/nmr/MPNMR.html, the site
maintained by Dror E. Warschawski, the determination of only a dozen has resorted to bicelles, often
in combination with other media (see Warschawski et al. 2011). The molecular mass of the largest of
those (cytochrome b 5 ) does not exceed 13 kDa. To the outside observer, a rapid survey of the
combined use of bicelles and NMR to study MP structure leaves the impression that establishing de
novo a structure using only data collected with bicelles is quite difficult and not the strongest point of
the approach. Indeed, despite 25 years of work on bicelles, all solution-NMR MP structures whose
monomers comprise more than a couple of TM segments have been obtained in detergent solutions
(see www.drorlist.com/nmr/MPNMR.html; reviewed by Marcotte and Auger 2005; Poget and Girvin
2007; Kim et al. 2009; Nietlispach and Gautier 2011; Warschawski et al. 2011). It is probably telling
that the first NMR structure of a G protein-coupled receptor (GPCR) was derived from solid-state
NMR data collected on liposome-reconstituted preparations rotated at the magic angle (Park et al.
2012), even though the group that carried out this study, that of Stanley J. Opella, has been a major
contributor to the development of bicelle technology (see e.g. Park et al. 2006, 2011a, c).
Bicelle-based NMR seems of greater use in a host of situations where it permits to complement
and extend data collected either by solution and/or solid-state NMR in other media, crystallography, or
electron microscopy (EM). Because bicelles comprise a bilayer region, in which MPs preferentially
reside (cf. Lee et al. 2008), they can be used to examine whether the protein structure is or not affected
by a purely detergent environment, a procedure often resorted to (see e.g. Fanucci et al. 2003; Howell
et al. 2005; Poget et al. 2007; Poget and Girvin 2007; Lau et al. 2008; Gautier et al. 2010), and to
determine the tilt of α-helices or β-barrels with respect to the bilayer plane (see e.g. Lindberg et al.
2003; De Angelis et al. 2004; Howell et al. 2005; Triba et al. 2006; Mahalakshmi and Marassi 2008),
an information that is generally missing in X-ray and EM structures and always absent from solutionNMR ones. Bicelles are also useful for examining conformational transitions that may not be easily
accessible to crystallography and may be perturbed by detergents (see e.g. Morrison et al. 2011;
Gustavsson et al. 2012) or for studying the consequences of the binding of drugs to MPs in a more
physiological environment than a detergent belt can provide (see e.g. Cui et al. 2010; Park et al.
2011b). They also provide an opportunity to study MP/lipid interactions and their effects on MP
function, structure, and oligomerization under much better conditions than in detergent solutions.
Furthermore, the fact that they permit to study the same MP in a continuous series of environments that
range from pure detergent solution to isotropic bicelles, both of them accessible to solution NMR, and
to aligned large bicelles and sheets or vesicles, which are a medium of choice for solid-state NMR
(Fig. 3.3), offers a unique opportunity to check and complete the information gathered using one
approach by that collected using another. This is an edge that bicelles have over nanodiscs, which, as
will be discussed in § 3.3, present a very interesting alternative when it comes to studying MPs in a
lipid bilayer environment by solution NMR but whose size can be adjusted only between rather narrow
limits.
3.2.2
Bicelles and Membrane Protein Crystallography
It is somewhat ironic that, while it is their use for NMR studies that provided the primary impetus to the
development of bicelles as hosts for MPs, the most important contribution bicelles have made to
structural studies of polytopic MPs is in the field of crystallography. Indeed, in the early 1990s, Salem
3.2 Bicelles
103