Outer membrane receptor
{{Pfam_box
| Symbol = TonB_dep_Rec
| Name = TonB dependent receptor
| image =1qfg_opm.png
| width =250
| caption = Structure of ferric hydroxamate uptake receptor.{{cite journal |vauthors=Ferguson AD, Welte W, Hofmann E, etal |title=A conserved structural motif for lipopolysaccharide recognition by procaryotic and eucaryotic proteins |journal=Structure |volume=8 |issue=6 |pages=585–92 |date=June 2000 |pmid=10873859 |doi= 10.1016/S0969-2126(00)00143-X|url=http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-41177|doi-access=free }}
| Pfam= PF00593
| Pfam_clan = CL0193
| InterPro= IPR000531
| SMART=
| PROSITE = PDOC00354
| SCOP = 2fcp
| TCDB = 1.B.14
| CDD = cd01347
| OPM family= 33
| OPM protein= 1qfg
| PDB=
}}
Outer membrane receptors, also known as TonB-dependent receptors, are a family of beta barrel proteins named for their localization in the outer membrane of gram-negative bacteria. TonB complexes sense signals from the outside of bacterial cells and transmit them into the cytoplasm, leading to transcriptional activation of target genes.
TonB-dependent receptors in gram-negative bacteria are associated with the uptake and transport of large substrates such as iron siderophore complexes and vitamin B12.{{cite journal|last1=Koebnik|first1=Ralf|title=Structures and function of bacterial outer membrane proteins: barrels in a nutshell|journal=MicroReview|date=2000|volume=37|issue=2|pages=239–253}}
TonB interactions with other proteins
In Escherichia coli, the TonB protein interacts with outer membrane receptor proteins that carry out high-affinity binding and energy-dependent uptake of specific substrates into the periplasmic space.{{cite journal |vauthors=Kadner RJ, Chimento DP, Wiener MC |title = The Escherichia coli outer membrane cobalamin transporter BtuB: structural analysis of calcium and substrate binding, and identification of orthologous transporters by sequence/structure conservation|journal = J. Mol. Biol.|volume = 332|issue = 5|pages = 999–1014|year = 2003|pmid = 14499604|doi = 10.1016/j.jmb.2003.07.005}} These substrates are either poorly transported through non-specific porin channels or are encountered at very low concentrations. In the absence of TonB, these receptors bind their substrates but do not carry out active transport. TonB-dependent regulatory systems consist of six protein protein components.{{cite journal |author = Koebnik R|title = TonB-dependent trans-envelope signalling: the exception or the rule?|journal = Trends Microbiol.|volume = 13|issue = 8|pages = 343–7|year = 2005|pmid = 15993072|doi = 10.1016/j.tim.2005.06.005}}
The proteins that are currently known or presumed to interact with TonB include BtuB,{{cite journal |vauthors=Kadner RJ, Chimento DP, Wiener MC, Mohanty AK |title=Substrate-induced transmembrane signaling in the cobalamin transporter BtuB |journal=Nat. Struct. Biol. |volume=10 |issue=5 |pages=394–401 |year=2003 |pmid=12652322 |doi=10.1038/nsb914|s2cid=24883519 }} CirA, FatA, FcuT, FecA,{{cite journal |vauthors=Deisenhofer J, Smith BS, Esser L, Chakraborty R, van der Helm D, Ferguson AD |title=Structural basis of gating by the outer membrane transporter FecA |journal=Science |volume=295 |issue=5560 |pages=1715–1719 |year=2002 |pmid=11872840 |doi=10.1126/science.1067313|bibcode=2002Sci...295.1715F |s2cid=86844549 }} FhuA,{{cite journal |vauthors=Moras D, Rosenbusch JP, Mitschler A, Rees B, Locher KP, Koebnik R, Moulinier L |title=Transmembrane signaling across the ligand-gated FhuA receptor: crystal structures of free and ferrichrome-bound states reveal allosteric changes |journal=Cell |volume=95 |issue=6 |pages=771–778 |year=1998 |pmid=9865695 |doi=10.1016/S0092-8674(00)81700-6|s2cid=16899072 |doi-access=free }} FhuE, FepA,{{cite journal |vauthors=Deisenhofer J, Xia D, Buchanan SK, Smith BS, Venkatramani L, Esser L, Palnitkar M, Chakraborty R, van der Helm D |title=Crystal structure of the outer membrane active transporter FepA from Escherichia coli |journal=Nat. Struct. Biol. |volume=6 |issue=1 |pages=56–63 |year=1999 |pmid=9886293 |doi=10.1038/4931|s2cid=20231287 }} FptA, HemR, IrgA, IutA, PfeA, PupA, LbpA and TbpA. The TonB protein also interacts with some colicins. Most of these proteins contain a short conserved region at their N-terminus.{{cite journal |author=Klebba PE |title=Three paradoxes of ferric enterobactin uptake |journal=Front. Biosci. |volume=8 |issue= 6|pages=s1422–s1436 |year=2003 |pmid=12957833 |doi=10.2741/1233|url=https://www.bioscience.org/2003/v8/s/1233/fulltext.htm|doi-access=free }}
TonB-dependent receptor plug domain
{{Pfam_box
| Symbol = Plug
| Name = TonB-dependent Receptor Plug Domain
| image =
| width =250
| caption =
| Pfam= PF07715
| InterPro= IPR012910
| SMART=
| Prosite =
| SCOP = 1fi1
| TCDB =
| OPM family=
| OPM protein=
| PDB=
}}
TonB-dependent receptors include a plug domain, an independently folding subunit that acts as the channel gate, blocking the pore until the channel is bound by ligand. At this point it undergoes conformational changes, opening the channel.{{cite journal |vauthors=Buchanan SK, Evans RW, Ghirlando R, Oke M, Sarra R, Farnaud S, Gorringe AR |title=The plug domain of a neisserial TonB-dependent transporter retains structural integ rity in the absence of its transmembrane beta-barrel |journal=FEBS Lett. |volume=564 |issue=3 |pages=294–300 |year=2004 |pmid=15111112 |doi=10.1016/S0014-5793(04)00196-6|s2cid=20056753 |doi-access=free |bibcode=2004FEBSL.564..294O }}
TonB as phage receptor
TonB also acts as a receptor for Salmonella bacteriophage H8. In fact, H8 infection is TonB dependent.{{Cite journal
| last1 = Rabsch | first1 = W.
| last2 = Ma | first2 = L.
| last3 = Wiley | first3 = G.
| last4 = Najar | first4 = F. Z.
| last5 = Kaserer | first5 = W.
| last6 = Schuerch | first6 = D. W.
| last7 = Klebba | first7 = J. E.
| last8 = Roe | first8 = B. A.
| last9 = Laverde Gomez | first9 = J. A. L.
| last10 = Schallmey
| doi = 10.1128/JB.00437-07 | first10 = M.
| last11 = Newton | first11 = S. M. C.
| last12 = Klebba | first12 = P. E.
| title = FepA- and TonB-Dependent Bacteriophage H8: Receptor Binding and Genomic Sequence
| journal = Journal of Bacteriology
| volume = 189
| issue = 15
| pages = 5658–5674
| year = 2007
| pmid = 17526714
| pmc =1951831
}}