Lipid-mediated interactions between intrinsic membrane proteins: dependence on protein size and lipid composition.

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 ( 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 A radius, corresponding roughly to one aliphatic chain, a medium cylinder of 5 A radius, corresponding to one alpha-helix, and a larger cylinder of 9 A 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 alpha-helices and depends on the lipid composition of the membrane.