The FadL family: unusual transporters for unusual substrates

The FadL family of proteins is responsible for the transport of hydrophobic compounds across the bacterial outer membrane. Two crystal structures of FadL, the long-chain fatty acid transporter from Escherichia coli, were recently determined, showing a novel fold characterized by the combination of a 14-stranded beta barrel and a "hatch" domain that plugs the barrel. Both crystal forms have several bound detergent molecules in the interior of the protein. This, together with differences between the N-terminal conformations of the FadL structures, has led to the proposal of a transport model that is distinct from those of all other known outer membrane transporters. According to this model, the transport of hydrophobic substrates across the outer membrane, as mediated by FadL family members, is based on diffusion, coupled to spontaneous conformational changes in the hatch domain.