A three-dimensional molecular assembly model of a lipoprotein from the Escherichia coli outer membrane.

From the complete amino-acid sequence of a lipoprotein from the outer membrane of E. coli, a three-dimensional molecular assembly model was constructed. It is proposed that the model provides a tubular hydrophilic channel through the outer membrane, which serves as a passive diffusion pore. An alpha-helix is constructed from the sequence, and six of them are arranged to form a superhelix with a hydrophilic interior and hydrophobic outer surface. The superhelical assembly is stabilized by seven ionic interactions between adjacent alpha-helices. Since the height of the assembly is 76 A, it could be inserted into the outer membrane and span the full 75-A thick membrane. The assembly is stabilized in the outer membrane not only by hydrophobic interaction between the surface of the assembly and the lipid bilayer, but also by three hydrocarbon chains of fatty acids linked to the amino-terminal end of the lipoprotein, which are flipped back along the assembly and inserted into the lipid bilayer of the outer membrane. Any two alpha-helices in an assembly are linked to the peptidoglycan at their carboxyl-terminal ends so that the outer membrane is anchored on the peptidoglycan layer. Six or more alpha-helices can form an assembly of this type. However, assuming that an assembly consists of six helices, there are 1.25 x 10(5) per cell hydrophilic channels of a diameter of 12.5 A and 35% of the cell surface is occupied by the assemblies.