|Gene:||tar||Accession Numbers: EG10988 (EcoCyc), b1886, ECK1887|
Synonyms: cheM, MCP-II, aspartate chemoreceptor protein, Tar dimer, chemotaxis signaling protein II
Component of: chemotaxis signaling complex - aspartate sensing (extended summary available)
Subunit composition of methyl accepting chemotaxis protein Tar = [Tar]2
The tar gene product is one of four methyl-accepting chemotaxis proteins (MCPs) in E. coli K-12. MCP-II is the receptor for the attractants aspartate and maltose and the repellents nickel and cobalt [Reader79, Wang80, Springer77]. Tar is the primary receptor for the amino acids aspartate, asparagine and glutamate; in strains lacking the Tsr receptor, Tar can mediate a chemotactic response to serine, cysteine, glycine, alanine and asparagine [Yang15]. Tar is responsible for mediating an attractant response to phenol [Imae87] and Tsr and Tar mediate a chemotactic response to changes in pH [Krikos85].
E. coli Tar is a homodimeric inner membrane protein; the Tar monomer consists of a periplasmic, ligand-sensing domain, two trans-membrane segments (TM1 and TM2) and a cytoplasmic signaling domain which is predominantly alpha-helical in structure and is predicted to contain 4 methylation sites [Lynch91, Bowie95, Le96]. The cytoplasmic domain of Tar is subject to methylation and demethylation at the carboxyl groups of glutamic acid residues [DeFranco80, Engstrom80, Krikos83]. Methylation and demethylation of MCPs in E. coli K-12 is catalysed by the CheR methyltransferase and the CheB methylesterase.
Aspartate binds directly to the Tar receptor whereas maltose detection is mediated via the periplasmic maltose binding protein [Manson85, Wolff88, Mowbray87, Gardina92, Gardina97, Gardina98, Zhang99]. Aspartate binding to a purified Tar receptor generates a downward piston motion of TM1 relative to TM2 [Ottemann99]
The cytoplasmic domains of the four E. coli MCPs have a high degree of sequence similarity [Krikos83, Le96, Alexander07]. Tar contains a HAMP domain (present in histidine kinases, adenylate cyclases, methyl accepting chemotaxis proteins, phosphatases) which is located between the transmembrane region of the molecule and the cytoplasmic signalling region. Tsr HAMP domains have been shown to mediate input/ouptut signaling [Ames08, Zhou09, Zhou11, Ames13, Samanta15] (and reviewed in [Parkinson10]).
E. coli MCPs form ternary complexes with the cytoplasmic proteins CheA and CheW [Gegner92]. Tar and Tsr are considered to be high-abundance receptors while Tap and Trg are low-abundance [Hazelbauer81, Hazelbauer81a, Harayama82].
Novel chemoeffectors specific for Tar have been identified as have two antagonistic compounds which bind to Tar but do not induce a chemotactic response [Bi13].
tar: taxis to aspartate and from repellents
Locations: inner membrane
|Map Position: [1,969,054 <- 1,970,715] (42.44 centisomes, 153°)||Length: 1662 bp / 553 aa|
Molecular Weight of Polypeptide: 59.944 kD (from nucleotide sequence), 60.0 kD (experimental) [Wang80 ]
Unification Links: ASAP:ABE-0006290 , CGSC:122 , DIP:DIP-10956N , DisProt:DP00294 , EchoBASE:EB0981 , EcoGene:EG10988 , EcoliWiki:b1886 , ModBase:P07017 , OU-Microarray:b1886 , PortEco:tar , PR:PRO_000024026 , Pride:P07017 , Protein Model Portal:P07017 , RefSeq:NP_416400 , RegulonDB:EG10988 , SMR:P07017 , String:511145.b1886 , Swiss-Model:P07017 , UniProt:P07017
Relationship Links: InterPro:IN-FAMILY:IPR003122 , InterPro:IN-FAMILY:IPR003660 , InterPro:IN-FAMILY:IPR004089 , InterPro:IN-FAMILY:IPR004090 , InterPro:IN-FAMILY:IPR004091 , PDB:Structure:2ASR , PDB:Structure:2L9G , Pfam:IN-FAMILY:PF00015 , Pfam:IN-FAMILY:PF00672 , Pfam:IN-FAMILY:PF02203 , Prints:IN-FAMILY:PR00260 , Prosite:IN-FAMILY:PS00538 , Prosite:IN-FAMILY:PS50111 , Prosite:IN-FAMILY:PS50885 , Smart:IN-FAMILY:SM00283 , Smart:IN-FAMILY:SM00304 , Smart:IN-FAMILY:SM00319
|Biological Process:||GO:0006935 - chemotaxis
[UniProtGOA11a, GOA01a, Silverman77]
GO:0007165 - signal transduction [UniProtGOA11a, GOA01a]
|Molecular Function:||GO:0004871 - signal transducer activity
[UniProtGOA11a, GOA01a, Springer77]
GO:0004888 - transmembrane signaling receptor activity [GOA01a, Springer77]
GO:0005515 - protein binding [Arifuzzaman06, Rajagopala09]
GO:0043424 - protein histidine kinase binding [Gegner92]
|Cellular Component:||GO:0005887 - integral component of plasma membrane
[Ridgway77, Krikos83, Lynch91]
GO:0005886 - plasma membrane [UniProtGOA11, UniProtGOA11a]
GO:0016020 - membrane [UniProtGOA11a, GOA01a]
GO:0016021 - integral component of membrane [UniProtGOA11a, GOA01a]
|MultiFun Terms:||cell processes → motility, chemotaxis, energytaxis (aerotaxis, redoxtaxis etc)|
|cell structure → membrane|
|regulation → type of regulation → posttranscriptional → inhibition / activation of enzymes|
|Growth Medium||Growth?||T (°C)||O2||pH||Osm/L||Growth Observations|
|LB enriched||Yes||37||Aerobic||6.95||Yes [Gerdes03, Comment 1]|
|LB Lennox||Yes||37||Aerobic||7||Yes [Baba06, Comment 2]|
|M9 medium with 1% glycerol||Yes||37||Aerobic||7.2||0.35||Yes [Joyce06, Comment 3]|
|MOPS medium with 0.4% glucose||Yes||37||Aerobic||7.2||0.22||Yes [Baba06, Comment 2]|
Subunit of: chemotaxis signaling complex - aspartate sensing
Synonyms: MCP-II signaling complex
Subunit composition of
chemotaxis signaling complex - aspartate sensing = [(CheA)2][CheW]2[(Tar)2]3
CheA(L) histidine kinase = (CheA)2
methyl accepting chemotaxis protein Tar = (Tar)2 (extended summary available)
The tar gene product is one of four methyl-accepting chemotaxis proteins (MCPs) in E. coli. MCP-II is the receptor for the attractant L-aspartate and related amino acids and dicarboxylic acids. MCP-II also interacts with the periplasmic maltose-binding protein to mediate taxis to the attractant maltose. It also responds to the repellents cobalt and nickel and is thermosensitive. [Nara96, Gardina97, Gardina92, Krikos83, Wang80, Chi97, Hoch95, Neidhardt96, Salman07, Jiang09, Wheatley15].
Chemotaxis in Escherichia coli is accomplished with a modified two-component signal transduction complex which transmits controlling signals to the flagellar motor complex. E.coli has four methyl-accepting chemotaxis protein (MCP)-type receptor complexes which recognize the following ligands: Tsr, serine; Tar, aspartate and maltose; Trg, ribose, galactose and glucose and Tap, dipeptides. Serine and aspartate bind directly to the receptor whereas maltose, ribose, galactose, glucose and dipeptides bind first to a periplasmic binding protein which then docks with its individual membrane receptor (reviewed in [Manson98]).
The receptor complexes are ternary structures. The receptor-ligand interaction domain is located in the periplasm. Each receptor serves as the organizational framework for a receptor kinase signaling supermolecular complex formed in conjunction with histidine kinase CheA and other components of the signaling pathway (reviewed in [Falke97]). There are two transmembrane (TM) linker domains (CheW) which couple the methylation-dependent receptor to CheA. The receptors form homodimers with or without ligands [Gegner92]. CheA is a histidine kinase capable of autophosphorylation using ATP as a phosphodonor. The receptor complex dimers form trigonal units which in turn form a two-dimensional hexagonal lattice [Shimizu00] located usually at one pole of the cell. The Tsr and Tar receptors are the most abundant and the Tap, Trg receptors are less prevalent [Bren00].
CheA and CheY comprise a two-component signal transduction system where the signal is transmitted via phosphorylation from CheA to CheY (the response regulator). In several ways CheA/CheY differs from the standard two-component paradigm. Most significantly, CheY does not possess a DNA-binding domain and it doesn't act as a transcription factor. In the absence of activator ligand, CheA autophosphorylation is stimulated thus increasing the phosphotransfer from CheA to CheY, the messenger protein. CheY-P has a lower affinity for CheA than CheY, resulting in the dissociation of CheY-P from CheA. CheY-P has a higher affinity than CheY for the flagellar motor protein, FliM, a component of the motor supramolecular complex [Welch93]. Binding of CheY to FliM increases the probability of flagellar rotation in the CW direction [Barak92]. CCW rotation of the motor induces the flagellar filaments to coalesce into a bundle which propels the cell forward in a fairly straight line (run). CW rotation disrupts the bundle and causes the cell to tumble. The cell typically travels in a three-dimensional walk consisting of runs interspersed with random chaotic tumbling. CheZ is a cytosolic phosphatase which prevents overaccumulation of CheY-P by accelerating the decay of its aspartyl-phosphate residue [Hess87]. CheY-P is thus maintained during steady-state conditions at a level that generates the random walk [Manson98].
When an attractant molecule binds to the receptor, a conformational change is induced [Yeh93] which propagates across the membrane and results in a suppression of CheA autophosphorylation. Levels of CheY-P decrease and the cells tumble less frequently, causing an increase in their run lengths as they enter areas of higher attractant concentrations. The adaptation response is necessary, though, for the cells to respond properly to continually increasing attractant concentration. Adaptive methylation is carried out by two enzymes: the methyltransferase CheR and the methylesterase CheB [Toews79]. CheR is a constitutive enzyme which, through the use of S-adenosylmethionine, methylates glutamate residues in the cytoplasmic domains of the MCPs. CheB is a target for phosphotransfer from CheA, and the activated CheB-P functions as a methyl esterase which removes methyl groups from the MCPs, reducing their kinase activity. Under steady-state conditions, the addition of methyl groups by CheR is balanced by the methyl group removal by CheB-P and an intermediate level of receptor methylation is maintained, resulting in run-tumble behavior of the cell. When an attractant binds to a receptor and inhibits CheA activity, the levels of CheB-P drop. The decrease is slower than that for CheY-P though, since CheB-P is not a phosphate donor to CheZ. The rising level of methyl esters eventually stimulate histidine kinase activity and therefore counteract the effect of attractant binding to the receptor. This resets the receptor signal to its basal level [Falke97].
The components of the chemotaxis sensory system are arranged at one of the cell poles in tight clusters containing thousands of copies of each protein [Sourjik00]. Binding of an attractant results in an increase in the probability that CheA is inactive (unphosphorylated) and methylation of CheA on four specific glutamate residues increases the probability that that it is active (phosphorylated) [Borkovich92]. Lower levels of methylation reduce the activity of CheA but increase the affinity of the receptor for its attractant ligand [Li00a].
|Transmembrane-Region||7 -> 33|
|Protein-Segment||64 -> 73|
|Transmembrane-Region||191 -> 211|
|Conserved-Region||214 -> 266|
|Conserved-Region||271 -> 500|
10/20/97 Gene b1886 from Blattner lab Genbank (v. M52) entry merged into EcoCyc gene EG10988; confirmed by SwissProt match.
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