Biomedical Engineering Reference
In-Depth Information
some biophysical techniques. The shape of detergents consisting of a bulky,
hydrophilic headgroup and a short-chain, hydrophobic tail results in high
lateral pressure in the region of the headgroup and low lateral pressure around
the tail, due to reduced steric clashes in the latter region. Such a profile is
opposite to that found in the membrane bilayer, where there are considerable
steric clashes between the long-chain lipid tails in the centre of the membrane.
Consequently, the natural curvature of a lipid bilayer leaflet is with the tails
facing outwards, opposite to that found in micelles. Formation of a flat bilayer
surface frustrates this natural lipid curvature, leading to a considerable
increase in lateral pressure in the centre of the membrane.
132-134
It has been
found that this lateral pressure profile is important for the functionality of
some membrane proteins
113
and is expected to have an effect on the structure
of others. Furthermore, the hydrophobic region of detergent micelles may be
less well defined than in a bilayer potentially expanding to accommodate
membrane proteins,
135
although this is not observed in all studies.
19,106
In order to circumvent a number of the problems associated with the use of
detergent micelles, bicelles and nanolipoprotein particles (nanodiscs) have been
proposed. Both consist of a bounded bilayer of lipids which, to some extent,
can be adjusted in size.
12.3.5 Nanolipoprotein Particles
Nanolipoprotein particles, commonly referred to as nanodiscs, consist ofa
bilayer of lipids enclosed by an amphipathic 'membrane scaffold protein'
(MSP) [Figure 12.2(b)] (reviewed in Borch and Hamann
136
). The scaffold
proteins are based on engineered clones of the apoplipoprotein AI class of
membrane scaffold proteins e.g., ApoE which is involved in high-density
lipoprotein-mediated reverse cholesterol transport in the body.
137,138
Structural studies demonstrate that nanodiscs are enveloped by two MSP
molecules, the most likely conformation being the 'double belt model'.
137,139
A
variety of different-sized MSP proteins have been engineered by adding or
removing helices allowing close control of the nanodiscs' size.
140
Assembly of the discs is described in detail elsewhere:
138,141
assembly is
initiated by addition of detergent-adsorbing beads in the presence of detergent-
solubilised membrane protein, scaffold protein and cholate-solubilised lipid.
The resulting nanodiscs can be purified for size homogeneity using HPLC
fractionation.
The ability to direct the nanodisc size enables close control of the state of the
embedded membrane protein; for example bacteriorhodopsin has been
embedded both as a monomer and a trimer in nanodiscs by varying the size
of scaffold protein and thus the lipid bilayer diameter.
24,141,142
This also
reduces adverse protein-protein interactions, minimising aggregation. The
amphipathic scaffold protein makes the discs soluble in detergent-free solution.
Similar to a native membrane environment, access to both sides of the bilayer
is possible.
