FhuA (TonA), the Career of a Protein

Abstract

The interest in the TonA protein, now called FhuA, has endured since the beginnings of phage genetics and molecular biology. The functions of this energy-coupled transporter and receptor in the outer membrane of Escherichia coli cells are fascinating. The historical perspective of this protein presented here is not intended to provide a comprehensive account of the structure and function of FhuA but rather attempts to explain how this protein has attracted interest for so long.The seminal work published by Salvador E. Luria and Max Delbrück (51) in 1943 is considered the beginning of modern bacterial genetics (12). In this publication, they reported their fluctuation test, which demonstrates that in bacteria, genetic mutations arise in the absence of selection rather than as a response to selection. They isolated virus-resistant mutants of Escherichia coli B, ascribed this virus resistance to mutations in the host cells prior to infection by the phage, and presented a means for quantitatively calculating mutation rates. The virus they used was later named phage T1, and the mutations that conferred resistance were subsequently designated tonA and tonB (ton derived from T one). It was later found that a phage morphologically and serologically distinct from T1, named T5, cannot infect tonA mutants but can infect tonB mutants; i.e., phage T5 requires only the tonA-encoded function but not the tonB-encoded function.Members of the "phage group" surrounding Luria and Delbrück were interested in genetics, reproduction, and multiplication. They studied, as the most simple system, phages and E. coli host cells, mutants, phage morphology, serology, adsorption, lysis, and burst size, but not biochemistry (12,14). The prejudice against biochemistry began to dissolve during the postdoctoral studies of Wolfhard Weidel, a biochemist by training, on phage adsorption and infection in Delbrück's laboratory. Delbrück's enthusiasm for Weidel's studies was evident in his report to the Caltech president, in which he referred to Weidel's work as "the most startling finding of the year" (66). Weidel's findings were included in the 1950 report on viruses of the Division of Biology of the California Institute of Technology. They were published in 1951 (68) after he was back in Germany at the Max Planck Institute of Biology in Tübingen, where he continued his work on phage adsorption, first with phages T2, T4, and T6 and then mainly with phage T5. His first achievement was the isolation of membrane fractions from E. coli B that bound the T phages with kinetics resembling those obtained with living bacteria. He could differentiate between receptors for T2, T4, and T6 on the one side and phage T5 on the other side. Electron microscopy revealed phage-induced degradation of the cell envelopes (68), which was later assigned to the T2 lysozyme (69).The methods used at that time were rather crude. The T5 receptor was extracted from cells by treatment with 0.1 N NaOH, neutralized with CO 2 , precipitated with 20% acetic acid, and then solubilized ...

Legionella pneumophila LbtU Acts as a Novel, TonB-Independent Receptor for the Legiobactin Siderophore

“…However, the slight differences between the three mutants at the highest CFU concentrations in panel C were not observed in all replicate experiments. minal 22-stranded ␤-barrel that forms a gated porin across the membrane and a long (ϳ150-residue) N-terminal periplasmic domain that acts as a "plug" that opens, with the assistance of TonB-ExbBD, when the receptor is engaged and the siderophore is to be imported (14,28,48,88,92,100,103,117,136). For comparison to LbtU, the secondary structures of two wellknown receptors appear in Fig.…”

Siderophores for molecular imaging applications

“…Due to the indispensability of siderophore-mediated iron acquisition, this system is hijacked during microbial competition, e.g. the outer membrane ferrichrome-type siderophore receptor of E. coli serves also as receptor for various bacteriophages [22] and naturally evolved siderophore-antibiotic conjugates, termed sideromycins, in which a bactericidal warhead is attached to a siderophore moiety [20,21]. For instance, albomycins comprise a hydroxamate siderophore unit, reminiscent of those found in fungal ferrichromes, and bactericidal unit that inhibits seryl-tRNA synthetase.…”

The Same Periplasmic ExbD Residues Mediate In Vivo Interactions between ExbD Homodimers and ExbD-TonB Heterodimers

“…These results suggested a model where ExbD 2 assembled with ExbB undergoes a transmembrane domain-dependent transition and exchanges partners in localized homodimeric interfaces to form an ExbD 2 -TonB heterotrimer. The findings here were also consistent with our previous hypothesis that ExbD guides the conformation of the TonB periplasmic domain, which itself is conformationally dynamic.The TonB system of Gram-negative bacteria couples the proton motive force (PMF) of the cytoplasmic membrane (CM) to the active transport of a diverse range of large, scarce, or important nutrients across the unenergized outer membrane (1,24,25). Active transport across the outer membrane requires TonB-gated transporters, which are 22-stranded ␤-barrels with lumens occluded by an amino-terminal globular domain called the cork (37).…”

“…The TonB system of Gram-negative bacteria couples the proton motive force (PMF) of the cytoplasmic membrane (CM) to the active transport of a diverse range of large, scarce, or important nutrients across the unenergized outer membrane (1,24,25). Active transport across the outer membrane requires TonB-gated transporters, which are 22-stranded ␤-barrels with lumens occluded by an amino-terminal globular domain called the cork (37).…”