Chemistry Reference
In-Depth Information
competing interactions with solvent detergent [
113
-
115
]. This can be exacerbated
by the frequent need to acquire data at elevated temperatures (35 to
40
C) [
112
].
Consequently, the identification of a detergent that offers an appropriate balance
between solubilization efficiency and preservation of native structure often requires
extensive screening and optimization of sample conditions. While a wide range of
detergents has proven useful for the study of membrane protein structure and
function, a surprisingly narrow selection has so far been used to solve the majority
of high-resolution membrane protein solution NMR structures (Table
1
). These
include micellar systems formed by sodium dodecyl sulfate (SDS), dodecylpho-
sphocholine (DPC), or short-chain phosphatidylcholines (C
6
-orC
7
-DHPC), and
small bicelles with C
6
-DHPC/DMPC (Table
1
).
>
3.1.1 SDS
SDS was one of the first detergents to be used for solution NMR of membrane
proteins [
116
,
117
], and has continued to be used for this purpose, particularly for
the study of smaller membrane proteins containing only a single TM or amphipathic
helix. SDS micelles have the ability to maintain a high concentration of membrane
proteins in solution while forming protein-detergent complexes with size and
dynamic properties that are often favorable for NMR spectroscopy. Although
SDS is generally classified as a denaturing detergent due to its tendency to unfold
water-soluble proteins [
114
,
118
], hydrophobic peptide segments that interact with
the micelle usually adopt a helical structure that minimizes exposure of polar
backbone atoms to the hydrophobic interior of the micelle in the same way that
would occur in lipid bilayers [
119
-
121
]. This has made this detergent a convenient
solvent for the study of isolated TM segments [
122
,
123
] and amphipathic mem-
brane surface binding helices (e.g., [
124
-
127
]).
In spite of the generally accepted classification of SDS as a denaturing detergent,
it has been widely used for the study of intermolecular interactions between isolated
TM helices [
123
,
128
,
129
] or multi-spanning membrane proteins connected by
short turn sequences [
130
,
131
]. The suitability of this detergent for the study of
TM-helix dimer structures has also been substantiated in a number of systems by
comparison of mutational effects on dimer affinity in SDS and bilayer
environments [
129
,
132
-
134
] although small discrepancies do occur in some
cases. However, it should be noted that examples of interactions identified in
SDS exist that did not appear in the structure of the full-length protein (e.g.,
DAGK [
135
]). Moreover, there is a potential for disruptive interactions to occur
between SDS and longer inter-helical loops or globular domains (e.g., the Y2
receptor [
75
]), making it necessary to adopt a cautious approach when interpreting
structural information for membrane proteins in SDS. To date only a handful of
larger (i.e., more than one TM segment) membrane proteins have been studied by
solution NMR in this detergent, although in all cases the structural data was well
validated by comparison with complimentary functional or structural data
[
136
-
138
].
