Transmembrane passage of hydrophobic compounds through a protein channel wall

Abstract

Hydrophobic compounds are transported both into and out of cells. Because of the lipophilic nature of hydrophobic compounds, it is thought that such substrates enter efflux pumps by lateral diffusion from the lipid bilayer. Here, Hearn and colleagues present the structure of an Escherichia coli long-chain fatty acid importer, FadL, which reveals an opening in the wall of its transmembrane β-barrel. They further show that a mutant in which this opening is constricted is unable to transport substrate. They conclude that importers, like exporters, may exploit lateral diffusion for the transport of hydrophobic substrates. In this paper Hearn et al. present the structure of an Escherichia coli long-chain fatty acid importer, FadL, which reveals an opening in the wall of its transmembrane beta-barrel. They further show that a mutant in which this opening is constricted is unable to transport substrate, and conclude that importers, like exporters, may exploit lateral diffusion for the transport of hydrophobic substrates. Membrane proteins that transport hydrophobic compounds have important roles in multi-drug resistance1,2,3 and can cause a number of diseases4,5, underscoring the importance of protein-mediated transport of hydrophobic compounds. Hydrophobic compounds readily partition into regular membrane lipid bilayers6, and their transport through an aqueous protein channel is energetically unfavourable3. Alternative transport models involving acquisition from the lipid bilayer by lateral diffusion have been proposed for hydrophobic substrates3,4,7,8,9,10,11,12. So far, all transport proteins for which a lateral diffusion mechanism has been proposed function as efflux pumps. Here we present the first example of a lateral diffusion mechanism for the uptake of hydrophobic substrates by the Escherichia coli outer membrane long-chain fatty acid transporter FadL. A FadL mutant in which a lateral opening in the barrel wall is constricted, but which is otherwise structurally identical to wild-type FadL, does not transport substrates. A crystal structure of FadL from Pseudomonas aeruginosa shows that the opening in the wall of the β-barrel is conserved and delineates a long, hydrophobic tunnel that could mediate substrate passage from the extracellular environment, through the polar lipopolysaccharide layer and, by means of the lateral opening in the barrel wall, into the lipid bilayer from where the substrate can diffuse into the periplasm. Because FadL homologues are found in pathogenic and biodegrading bacteria, our results have implications for combating bacterial infections and bioremediating xenobiotics in the environment.