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Biophys J, July 2001, p. 276-284, Vol. 81, No. 1

and
*Department of Chemistry, Université de Montréal,
succursale Centre-Ville, Montréal, Québec H3C 3J7, Canada;
Physics Department, McGill University, Montréal,
Québec H3C 3J7 Canada;
Department of Physics,
Simon Fraser University, Burnaby, British Columbia V5A 1S6 Canada;
§Department of Biochemistry, Weill Medical College of
Cornell University, New York, NY 10021 USA
The present study is an application of an approach
recently developed by the authors for describing the structure of the
hydrocarbon chains of lipid-bilayer membranes (LBMs) around embedded
protein inclusions (Lagüe et al., 2000 Biophys. J. 79:2867-2879). The approach is based on statistical mechanical
integral equation theories developed for the study of dense liquids.
First, the configurations extracted from molecular dynamics simulations
of pure LBMs are used to extract the lateral density-density response function. Different pure LBMs composed of different lipid molecules were considered: dioleoyl phosphatidylcholine (DOPC), palmitoyl-oleoyl phosphatidylcholine (POPC), dipalmitoyl phosphatidylcholine (DPPC), and
dimyristoyl phosphatidylcholine (DMPC). The results for the lateral
density-density response function was then used as input in the
integral equation theory. Numerical calculations were performed for
protein inclusions of three different sizes. For the sake of
simplicity, protein inclusions are represented as hard smooth cylinders
excluding the lipid hydrocarbon core from a small cylinder of 2.5 Å radius, corresponding roughly to one aliphatic chain, a medium cylinder
of 5 Å radius, corresponding to one
-helix, and a larger cylinder
of 9 Å radius, representing a small protein such as the gramicidin
channel. The lipid-mediated interaction between protein inclusions was
calculated using a closed-form expression for the
configuration-dependent free energy. This interaction was found to be
repulsive at intermediate range and attractive at short range for two
small cylinders in POPC, DPPC, and DMPC bilayers, whereas it oscillates
between attractive and repulsive values in DOPC bilayers. For medium
size cylinders, it is again repulsive at intermediate range and
attractive at short range, but for every model LBM considered here. In
the case of a large cylinder, the lipid-mediated interaction was shown
to be repulsive for both short and long ranges for the DOPC, POPC, and
DPPC bilayers, whereas it is again repulsive and attractive for DMPC
bilayers. The results indicate that the packing of the hydrocarbon
chains around protein inclusions in LBMs gives rise to a generic (i.e., nonspecific) lipid-mediated interaction which favors the association of
two
-helices and depends on the lipid composition of the membrane.
Biophys J, July 2001, p. 276-284, Vol. 81, No. 1
© 2001 by the Biophysical Society 0006-3495/01/07/276/09 $2.00
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