Fig. 1

Cartoon structural representations generated in Pymol showing the internal hydrophobic channels. a AlkL, b OmpW, and c FadL spanning the outer membrane. The coloured areas represent hydrophobic regions, including the internal hydrophobic channels leading to the gap in the β-sheet wall. In a model for the mechanism of lateral diffusion of small hydrophobic substrates through FadL: d extracellular substrates, in this case octane, bind sequentially to a low affinity binding site (L); e then a high affinity binding site (H), f leading to conformational change at the N-terminus (purple) of the plug. This creates a continuous channel through which the substrate can diffuse through to an opening caused by a kink in the S3 β-sheet strand and into the outer membrane