Outer-membrane transport of aromatic hydrocarbons as a first step in biodegradation

Bacterial biodegradation of hydrocarbons, an important process for environmental remediation, requires the passage of hydrophobic substrates across the cell membrane. Here, we report crystal structures of two outer membrane proteins, Pseudomonas putida TodX and Ralstonia pickettii TbuX, which have been implicated in hydrocarbon transport and are part of a subfamily of the FadL fatty acid transporter family. The structures of TodX and TbuX show significant differences with those previously determined for Escherichia coli FadL, which may provide an explanation for the substrate-specific transport of TodX and TbuX observed with in vivo transport assays. The TodX and TbuX structures revealed 14-stranded beta-barrels with an N-terminal hatch domain blocking the barrel interior. A hydrophobic channel with bound detergent molecules extends from the extracellular surface and is contiguous with a passageway through the hatch domain, lined by both hydrophobic and polar or charged residues. The TodX and TbuX structures support a mechanism for transport of hydrophobic substrates from the extracellular environment to the periplasm via a channel through the hatch domain.