Chemistry Reference
In-Depth Information
detergent-protein interactions to alter the structure. This has been suggested to be a
factor in the differences observed between some solid-state structures obtained in
lipid membranes and solution-state detergent-solubilized forms [
159
].
Another potential complication arising from the use of micelles is the significant
curvature of the micelle surface, a particularly relevant concern for surface-binding
elements. This is capable of inducing curvature in amphipathic helices to maximize
burial of the hydrophobic side of the helix in the hydrophobic phase of the micelle
[
160
]. Structural elements with an affinity for the polar regions of the native bilayer
could also follow the curved dimension of the headgroup phase, potentially leading
to distortions of adjacent parts of the structure (e.g., the voltage dependent potas-
sium channel KvAP [
161
]). Similarly, mismatch between the size of the hydropho-
bic core of the micelle and TM segment length can induce curvature in the TM helix
to minimize its exposure to the aqueous phase [
162
]. In addition, the less densely
packed, more dynamic state of the detergent monomers makes the hydrophobic
phase of the micelle easier to access compared to that of a lipid bilayer [
163
]. This
can promote detergent-protein interactions that may not occur in native lipid
membranes, potentially distorting and/or destabilizing other parts of the protein
[
164
,
165
].
An illustrative example of how detergents can influence structure is provided by
the controversy centered about the influenza A M2 channel. In C
6
-DHPC micelles
M2 was found in a tightly packed tetrameric state that bound the inhibitor
rimantidine at four allosteric sites in the lipid-facing part of the channel exterior,
inhibiting the channel by stabilizing the closed state [
166
]. In contrast, X-ray
crystallographic [
158
] as well as solid-state NMR data acquired in lipid bilayers
[
167
] on a shorter M2 construct clearly showed a single inhibitor-binding site inside
the channel, with no interactions at the exterior allosteric sites. Based on these
differences it was suggested that the helix tilt of the M2 TM segment in lipid
bilayers could be important in maintaining a more open, active channel conforma-
tion [
168
]. According to this idea the malleability of loosely packed detergent
molecules could allow coverage of a larger hydrophobic surface (Fig.
2d
),
eliminating the environment-induced tilt required to promote larger helix crossing
angles. However, functional evidence can be cited that supports the biological
relevance of both forms M2 channel [
158
,
166
,
169
], suggesting that the two
structure types represent different functional states for this protein. This hypothesis
is supported by the conformational dynamics observed for M2 in C
6
-DHPC [
166
]
and the structure of a drug-resistant mutant that had a reduced affinity for
rimantidine at the allosteric site [
170
]. Moreover, subsequent experiments have
shown that DPC micelles can support a tetrameric state that binds rimantidine at a
single site within the channel [
171
]. This confirms that a micellar environment is
capable of capturing the open state of this channel, although the physical properties
of the micelle that promote one form of the channel over the other remain one of the
interesting questions to be addressed.
