1.B.9 The FadL Outer Membrane Protein (FadL) Family

The FadL family includes several distantly related proteins, all probably outer membrane proteins, sequenced from E. coli, Haemophilus influenzae, Pseudomonas putida, Moraxella catarrhalis and Chlorobium limicola. The E. coli FadL protein functions in long chain fatty acid transport across the outer membrane. Residues involved in fatty acid binding and transport have been distinguished and identified. The XylN and TodX proteins of P. putida are encoded on TOL (toluene-degradation) plasmids and are concerned with transport of aromatic compounds such as toluene, m-xylene and benzyl alcohol. Other homologues are not characterized functionally.

Proteins of the FadL family are of about 450 amino acyl residues in length. They exhibit a single N-terminal hydrophobic sequence that may serve as the cleavable membrane-targeting signal sequence. The remainder of the proteins exhibit a preponderance of β-structure which forms β-barrels as do other bacterial porins. However, in contrast to most structurally characterized porins, FadL is monomeric.

The 3-dimensional structure of FadL has been solved at 2.6 Å resolution (van den Berg et al., 2004). It forms a monomeric 14-stranded β-barrel that is occluded by a central hatch domain. The hydrophobic compounds probably bind to multiple sites in the FadL channel and use a transport mechanism that involves spontaneous conformational changes in the hatch region. 

Hearn et al., 2009 presented an example of a lateral diffusion mechanism for the uptake of hydrophobic substrates by the Escherichia coli FadL. A FadL mutant, in which a lateral opening in the barrel wall was constricted, but otherwise unalterred, did not transport substrates. A crystal structure of FadL from Pseudomonas aeruginosa showed that the opening in the wall of the beta-barrel is conserved and delineates a long, hydrophobic tunnel that can mediate substrate passage from the extracellular environment, through the polar lipopolysaccharide layer. By means of the lateral opening in the barrel wall, substrates can pass into the lipid bilayer from where they can diffuse into the periplasm (Hearn et al., 2009).

Regular phospholipid bilayers do not pose efficient barriers for the transport of hydrophobic molecules. The outer membrane (OM) surrounding Gram-negative bacteria is a nontypical, asymmetric bilayer with an outer layer of lipopolysaccharide (LPS). The sugar molecules of the LPS layer prevent spontaneous diffusion of hydrophobic molecules across the OM. As regular OM channels such as porins do not allow passage of hydrophobic molecules, specialized OM transport proteins are required for their uptake. Such proteins, exemplified by channels of the FadL family, transport their substrates according to a lateral diffusion mechanism. Substrates diffuse from the lumen of the β-barrel laterally into the OM, through a stable opening in the wall of the barrel. In this way, the lipopolysaccharide barrier is bypassed, and, by depositing the substrates into the OM, a driving force for uptake is provided. Lateral diffusion through protein channel walls also occurs in alpha-helical inner membrane proteins, and could represent a widespread mechanism for proteins that transport and interact with hydrophobic substrates (van den Berg, 2010) .

The generalized transport reaction catalyzed by FadL family proteins is:

Hydrophobic compound (out) hydrophobic compound (periplasm)