Abstract
The stoichiometry of the first shell of lipids interacting with a transmembrane protein is defined operationally by the population of spin-labeled lipid chains whose motion is restricted directly by the protein. Interaction stoichiometries have been determined experimentally for a wide range of alpha-helical integral membrane proteins by using spin-label ESR spectroscopy. Here, we determine the spatially defined number of first-shell lipids at the hydrophobic perimeter of integral membrane proteins whose 3D structure has been determined by X-ray crystallography and lipid-protein interactions characterized by spin-labeling. Molecular modeling is used to build a single shell of lipids surrounding transmembrane structures derived from the PDB. Constrained energy optimization of the protein-lipid assemblies is performed by molecular mechanics. For relatively small proteins (up to 7-12 transmembrane helices), the geometrical first shell corresponds to that defined experimentally by perturbation of the lipid-chain dynamics. For larger, multi-subunit alpha-helical proteins, the lipids perturbed directly by the protein may either exceed or be less in number than those that can be accommodated at the intramembranous perimeter. In these latter cases, the motionally restricted spin-labeled lipids can be augmented by intercalation, or can correspond to a specific subpopulation at the protein interface, respectively. For monomeric beta-barrel proteins, the geometrical lipid stoichiometry corresponds to that determined from lipid mobility for a 22-stranded barrel, but fewer lipids are motionally restricted than can be accommodated around an eight-stranded barrel. Deviations from the geometrical first shell, in the beta-barrel case, are for the smaller protein with a highly curved barrel.