Long-chain fatty acid transporter (FadL)
Escherichia coli (bacteria)
Cells in prokaryotes and eukaryotes require various molecules, including fatty acids, from the environment to be transported into the cytoplasm for various important regulatory functions. Even though fatty acids can spontaneously cross membranes because of their hydrophobic nature, cells have evolved efficient, regulated systems for the transport of fatty acids across their membranes. Long chain fatty acids (LCFA) are long hydrophobic fatty acids with 11 to 18 carbon atoms. Bacteria use LCFAs from their growth medium or environment as a source of carbon and energy. In Gram-negative bacteria, LCFAs have to cross the outer membrane (OM) before they enter the periplasmic space and proceed to cross the plasma membrane into the cyotsol. Their transport across the OM and the plasma membrane is facilitated by various membrane proteins. FadL is a member of a conserved family of OM proteins and is involved in the transport of LCFAs across the OM.
The structure here is that of the FadL protein from E. coli. FadL forms a 14-stranded beta barrel which is plugged by a small hatch domain consisting of 42 amino acid residues from the N-terminus. The hatch domain closes the channel between the extracellular environment and the periplasm. FadL also has a distinctive kink at its extracellular end that results in a gap in the barrel wall. The LCFA molecule is bound to FadL, initially in a low-affinity groove formed by two extracellular loops, followed by transport to a high-affinity hydrophobic binding pocket at the extracellular end of the membrane. In the presence of a bound LCFA molecule, the hatch and the kink of FadL undergo a conformational change opening the channel, allowing the transport of the LCFA into the periplasm.
Protein Data Bank (PDB)
van den Berg, B. Black, P.N. Clemons Jr., W.M. Rapoport, T.A.; "Crystal structure of the long-chain fatty acid transporter FadL."; Science; (2004) 304:1506-1509 PubMed:15178802.
author: Ashwini Patil