Quantitative Comparison of GPCR Interactions with the Lipid Bilayer of the Cubic and Lamellar Mesophases

Abstract

Recent successes in the crystallographic determination of structures of transmembrane (TM) proteins in the G protein-coupled receptor (GPCR) family have established the lipidic cubic phase (LCP or “in meso”) environment as useful for growing diffraction quality crystals. The mechanism underlying in meso crystallogenesis is currently at a descriptive level. To begin developing a quantitative, energy-based nucleation and crystallization mechanism we are conducting molecular dynamics studies of the GPCR, rhodopsin, reconstituted into the LCP using the coarse-grained representations of the Martini force-field. The first aim is to quantify differences in the hydrophobic/hydrophilic exposure of the GPCR to lipids in the cubic and lamellar phases. Simulations of a single rhodopsin molecule in these monoolein-based mesophases showed more energetically unfavorable hydrophobic-hydrophilic interactions between the protein and lipid in planar bilayers of the lamellar phase. The reduced level of hydrophobic mismatch in the LCP, by contrast, is attributable to the highly curved geometry of the cubic phase that provides for more efficient shielding of the protein from unfavorable hydrophobic exposure. Since hydrophobic mismatch can drive oligomerization (Mondal et al., BJ 2011 - in press), these differences suggest that compared to the LCP, lamellar structures provide a more favorable setting in which GPCRs can oligomerize as a prelude to nucleation and crystal growth. These new findings lay the foundation for future studies of in meso crystallization mechanisms and for a rational approach to the generation of structure-quality crystals of membrane proteins.