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2.A.6.2.2
Multidrug/dye/detergent/bile salt/organic solvent resistance pump (substrates include: chloramphenicol, tetracycline, erythromycin, nalidixic acid, fusidic acid, fluoroquinolones, lipophilic β-lactams, norfloxacin, doxorubicin, novobiocin, rifampin, trimethoprim, acriflavin, crystal violet, ethidium, disinfectants, rhodamine-6G, TPP, benzalkonium, SDS, Triton X-100, deoxycholate/bile salts/organic solvents (alkanes), growth inhibitory steroid hormones (estradiola and progesterone), and phospholipids) (Elkins and Mullis, 2006). Lateral entry of substrates from the lipid bilayer into AcrB and its homologues has been proposed (Yu et al., 2003a; 2003b). [An asymmetric trimeric structure is established with AcrA having a hexameric structure, and TolC having a trimeric structure (Seeger et al., 2006]. A structure of a complex with YajC is also known (Törnroth-Horsefield et al., 2007). A covalently linked trimer of AcrB provides evidence for a peristaltic pump, alternative access, rotation mechanism (Takatsuka and Nikaido, 2009;Nikaido and Takatsuka, 2009; Pos, 2009) Further evidence for a rotatory mechanisms stems from kinetic analyses for cephalosporin efflux which can exhibit positive cooperativity (Nagano and Nikaido, 2009). May also export signaling molecules for cell-cell communication (Yang et al., 2006). The substrates may be captured in the lower cleft region of AcrB, then transported through the binding pocket, the gate, and finally to the AcrA funnel that connects AcrB to TolC (Husain & Nikaido et al., 2010).  AcrB has been converted into a light-driven proton pump using delta-rhodopsin (dR) linked to AcrB via a glycophorin A transmembrane domain. This created a solar powered protein capable of selectively capturing antibiotics from bulk solutions (Kapoor and Wendell 2013).  The trimeric structure is essential for activity (Ye et al. 2014).  Association with AcrZ (TC# 8.A.50), a small 1 TMS protein (49 aas) that modifies the substrate specificity of AcrAB, has been demonstrated (Hobbs et al. 2012).  In a similar way, the binding of YajC to AcrB stimulates the export of ampicillin (Törnroth-Horsefield et al. 2007). AcrZ binds to AcrB in a concave surface of the transmembrane domain (Du et al. 2015).  Substrate binding accelerates conformational transitions and substrate dissociation, demonstrating cooperativity (Wang et al. 2015). The overall structure of AcrAB-TolC exemplifies the adaptor bridging model, wherein the funnel-like AcrA hexamer forms an intermeshing cogwheel interaction with the alpha-barrel tip region of TolC. Direct interaction between AcrB and TolC is not allowed (Kim et al. 2015).  TMS2 in AcrB is required for lipophilic carboxylate binding. A groove shaped by the interface between TMS1 and TMS2 specifically binds fusidic acid and other lipophilic carboxylated drugs (Oswald et al. 2016). After ligand binding, a proton may bind to an acidic residue(s) in the transmembrane domain, i.e., Asp407 or Asp408, within the putative network of electrostatically interacting residues, which also include Lys940 and Thr978, and this may initiate a series of conformational changes that result in drug expulsion (Su et al. 2006). His978 is probably on the H+ pathway (Takatsuka and Nikaido 2006). AcrAB-TolC segregates to the old pole following cell division, causing the two daughter cells to exhibit different drug resistances (Bergmiller et al. 2017). The hoisting-loop is a highly flexible hinge that enables conformational energy transmission (Zwama et al. 2017). AcrB exhibits three distinct conformational states in the transport cycle, substrate access, binding, and extrusion, or loose (L), tight (T), and open (O) states, respectively (Yue et al. 2017). Simulations show that both Asp407 and Asp408 are deprotonated in the L/T states, while only Asp408 is protonated in the O state. Release of a proton from Asp408 in the O state results in large conformational changes.  Simulations offer dynamic details of how proton release drives the O-to-L transition in AcrB (Yue et al. 2017).  The three-dimensional structures of the homo-trimer complexes of AcrB-like transporters, and a three-step functional rotation helps to explain the mechanism of transport, but a more comprehensive model has been proposed (Zhang et al. 2017). Preparation of the trimeric complex (AcrAB/TolC) for cryo EM has been described (Du et al. 2018). The structural and energetic basis behind coupling functional rotation to proton translocation has been presented (Matsunaga et al. 2018). Protonation of  transmembrane Asp408 in the drug-bound protomer drives rotation. The conformational pathway identifies vertical shear motions among several transmembrane helices, which regulate alternate access of water as well as peristaltic motions that pump drugs into the periplasm (Matsunaga et al. 2018). CryoEM of detergent-free AcrB preserves lipid-protein interactions for visualization and reveals how the lipids pack against the protein (Qiu et al. 2018). In the presence of translation-inhibiting antibiotics, resistance acquisition depends on the AcrAB-TolC multidrug efflux pump, because it reduces tetracycline concentrations in the cell. Protein synthesis can thus persist and TetA expression can be initiated immediately after plasmid acquisition. AcrAB-TolC efflux activity can also preserve resistance acquisition by plasmid transfer in the presence of antibiotics with other modes of action (Nolivos et al. 2019).

Accession Number:P31224
Protein Name:AcrB aka ACRE aka B0462
Length:1049
Molecular Weight:113574.00
Species:Escherichia coli [83333]
Number of TMSs:12
Location1 / Topology2 / Orientation3: Cell inner membrane1 / Multi-pass membrane protein2
Substrate lipophilic beta-lactams, multiple drugs, detergents, bile salts, organic solvents, Disinfectants, dyes, steroid hormones, phospholipids, ethidium, lipophilic carboxylates

Cross database links:

Genevestigator: P31224
EchoBASE: EB1655
EcoGene: EG11704
eggNOG: COG0841
HEGENOM: HBG293864
DIP: DIP-9049N
RefSeq: AP_001111.1    NP_414995.1   
Entrez Gene ID: 945108   
Pfam: PF00873   
Drugbank: Drugbank Link   
BioCyc: EcoCyc:ACRB-MONOMER    ECOL168927:B0462-MONOMER   
KEGG: ecj:JW0451    eco:b0462   

Gene Ontology

GO:0016021 C:integral to membrane
GO:0005886 C:plasma membrane
GO:0005515 F:protein binding
GO:0005215 F:transporter activity
GO:0006810 P:transport

References (12)

[1] “Molecular cloning and characterization of acrA and acrE genes of Escherichia coli.”  Ma D.et.al.   8407802
[2] “The complete genome sequence of Escherichia coli K-12.”  Blattner F.R.et.al.   9278503
[3] “Highly accurate genome sequences of Escherichia coli K-12 strains MG1655 and W3110.”  Hayashi K.et.al.   16738553
[4] “Genes acrA and acrB encode a stress-induced efflux system of Escherichia coli.”  Ma D.et.al.   7651136
[5] “Molecular construction of a multidrug exporter system, AcrAB: molecular interaction between AcrA and AcrB, and cleavage of the N-terminal signal sequence of AcrA.”  Kawabe T.et.al.   10920254
[6] “Protein complexes of the Escherichia coli cell envelope.”  Stenberg F.et.al.   16079137
[7] “Global topology analysis of the Escherichia coli inner membrane proteome.”  Daley D.O.et.al.   15919996
[8] “Crystal structure of bacterial multidrug efflux transporter AcrB.”  Murakami S.et.al.   12374972
[9] “Structural basis of multiple drug-binding capacity of the AcrB multidrug efflux pump.”  Yu E.W.et.al.   12738864
[10] “Crystal structures of a multidrug transporter reveal a functionally rotating mechanism.”  Murakami S.et.al.   16915237
[11] “Structural asymmetry of AcrB trimer suggests a peristaltic pump mechanism.”  Seeger M.A.et.al.   16946072
[12] “Drug export pathway of multidrug exporter AcrB revealed by DARPin inhibitors.”  Sennhauser G.et.al.   17194213
Structure:
1IWG   1OY6   1OY8   1OY9   1OYD   1OYE   1T9T   1T9U   1T9V   1T9W   [...more]

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FASTA formatted sequence
1:	MPNFFIDRPI FAWVIAIIIM LAGGLAILKL PVAQYPTIAP PAVTISASYP GADAKTVQDT 
61:	VTQVIEQNMN GIDNLMYMSS NSDSTGTVQI TLTFESGTDA DIAQVQVQNK LQLAMPLLPQ 
121:	EVQQQGVSVE KSSSSFLMVV GVINTDGTMT QEDISDYVAA NMKDAISRTS GVGDVQLFGS 
181:	QYAMRIWMNP NELNKFQLTP VDVITAIKAQ NAQVAAGQLG GTPPVKGQQL NASIIAQTRL 
241:	TSTEEFGKIL LKVNQDGSRV LLRDVAKIEL GGENYDIIAE FNGQPASGLG IKLATGANAL 
301:	DTAAAIRAEL AKMEPFFPSG LKIVYPYDTT PFVKISIHEV VKTLVEAIIL VFLVMYLFLQ 
361:	NFRATLIPTI AVPVVLLGTF AVLAAFGFSI NTLTMFGMVL AIGLLVDDAI VVVENVERVM 
421:	AEEGLPPKEA TRKSMGQIQG ALVGIAMVLS AVFVPMAFFG GSTGAIYRQF SITIVSAMAL 
481:	SVLVALILTP ALCATMLKPI AKGDHGEGKK GFFGWFNRMF EKSTHHYTDS VGGILRSTGR 
541:	YLVLYLIIVV GMAYLFVRLP SSFLPDEDQG VFMTMVQLPA GATQERTQKV LNEVTHYYLT 
601:	KEKNNVESVF AVNGFGFAGR GQNTGIAFVS LKDWADRPGE ENKVEAITMR ATRAFSQIKD 
661:	AMVFAFNLPA IVELGTATGF DFELIDQAGL GHEKLTQARN QLLAEAAKHP DMLTSVRPNG 
721:	LEDTPQFKID IDQEKAQALG VSINDINTTL GAAWGGSYVN DFIDRGRVKK VYVMSEAKYR 
781:	MLPDDIGDWY VRAADGQMVP FSAFSSSRWE YGSPRLERYN GLPSMEILGQ AAPGKSTGEA 
841:	MELMEQLASK LPTGVGYDWT GMSYQERLSG NQAPSLYAIS LIVVFLCLAA LYESWSIPFS 
901:	VMLVVPLGVI GALLAATFRG LTNDVYFQVG LLTTIGLSAK NAILIVEFAK DLMDKEGKGL 
961:	IEATLDAVRM RLRPILMTSL AFILGVMPLV ISTGAGSGAQ NAVGTGVMGG MVTATVLAIF 
1021:	FVPVFFVVVR RRFSRKNEDI EHSHTVDHH