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3.5 Protein Dynamics
Transmission of signals between cells, within cells, and from the extracellular envi-
ronment to the cellular interior is essential to life, and the dynamic properties of
the signaling proteins are crucial to their functions [ 156 ]. Therefore, understanding
the dynamic nature of a membrane protein within lipid bilayers is crucial to reveal its
function mechanism at the molecular level.
SSNMR may be the best technique to study membrane protein dynamics in the
native lipid environment [ 56 , 157 , 158 ], and site specific 2 H labeling at the methyl
groups of Ala, Leu, and Val is most commonly used in SSNMR, and is an excellent
probe of the dynamics of membrane proteins. The Glaubitz group has recently
reported a dynamic picture of the green proteorhodopsin structure using through-
space and through-bond correlation experiments in SSNMR [ 41 ]. They have used
U-[ 13 C, 15 N]-PR and reversely labeled U-[ 13 C, 15 N] \WHYFI -PR samples to establish
a clear correlation between hydration water and the mobile J-residues (mainly in
flexible loops and tails), as indicated in Fig. 9 . Hydration water plays an essential
role for enhancing molecular dynamics of residues in tails and interhelical loops,
but not in transmembrane domains or rigid, structured loop segments. The result is
very important for understanding the dynamic proton pumping mechanism of
proteorhodopsin. It also provides an approach to study the site-resolved effects of
water and lipid bilayers on the dynamic properties of membrane proteins in general.
Fig. 9 13 C-DARR spectra of the aliphatic regions of U-[ 13 C, 15 N] \WHYFI -PR at 273 ( blue , gel
phase) and 313 K ( red , liquid crystal phase) (a) and the modified homology model of green PR (b).
Helical residues influenced by changes in membrane elasticity (labeled in blue ) are found in
helices C, E, F, and G as well as in loops EC and EF. These residues disappear in the fluid
membrane but are visible in the gel phase. This indicates that especially helices C and G but also E
and F undergo thermal equilibrium fluctuations in the ground state of PR. Adapted from [ 41 ] with
permission from the American Chemical Society
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