TonB-Dependent Transporter FhuA in Planar Lipid Bilayers: Partial Exit of Its Plug from the Barrel

• Published in: Biochemistry (Easton) 2012-08, Vol.51 (34), p.6753-6759
• Main Authors: Udho, Eshwar, Jakes, Karen S, Finkelstein, Alan
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Bacterial Outer Membrane Proteins - chemistry; Bacterial Outer Membrane Proteins - genetics; Bacterial Outer Membrane Proteins - metabolism; Cell Membrane - chemistry; Cell Membrane - genetics; Cell Membrane - metabolism; detergents; electron microscopy; Escherichia coli - chemistry; Escherichia coli - genetics; Escherichia coli - metabolism; Escherichia coli Proteins - chemistry; Escherichia coli Proteins - genetics; Escherichia coli Proteins - metabolism; gels; glycerol; Gram-negative bacteria; ligands; lipid bilayers; Lipid Bilayers - chemistry; Lipid Bilayers - metabolism; mass spectrometry; Membrane Proteins - genetics; Membrane Proteins - metabolism; Molecular Sequence Data; siderophores; staining; transporters; trypsin; urea; vitamin B12
• ISSN: 0006-2960; 1520-4995; 1520-4995
• DOI: 10.1021/bi300493u

TonB-dependent transporters (TBDTs), which transport iron-chelating siderophores and vitamin B12 across the outer membrane of Gram-negative bacteria, share a conserved architecture of a 22-stranded β-barrel with an amino-terminal plug domain occluding the barrel. We previously reported that we could induce TBDTs to reversibly open in planar lipid bilayers via the use of urea and that these channels were responsive to physiological concentrations of ligands. Here we report that in the presence of urea, trypsin can cleave the amino-terminal 67 residues of the plug of the TonB-dependent transporter FhuA, as assessed by gel shift and mass spectrometry assays. On the bilayer, trypsin treatment in the presence of urea resulted in the induced conductance no longer being reversed upon removal of urea, suggesting that urea opens intact FhuA channels by pulling the plug at least partly out of the barrel and that removal of the urea then allows reinsertion of the plug into the barrel. When expressed separately, the FhuA plug domain was found to be a mostly unfolded structure that was able to occlude isolated FhuA β-barrels inserted into the membrane. Thus, although folded in the barrel, the plug need not be folded upon exiting the barrel. The rate of insertion of the β-barrels into the membrane was tremendously increased in the presence of an osmotic gradient provided by either urea or glycerol. Negative staining electron microscopy showed that FhuA in a detergent solution formed vesicles, thus explaining why an osmotic gradient promoted the insertion of FhuA into membranes.