2.A.76 The Resistance to Homoserine/Threonine (RhtB) Family
Hundreds of sequenced proteins, derived from Gram-negative and Gram-positive bacteria as well as archaea, comprise the RhtB family, but few of these proteins are functionally characterized (Vrljic et al., 1999). E. coli possesses five paralogues, and a large region of one of them (YahN of E. coli) exhibits significant sequence similiarity to YggA of E. coli, an established member of the LysE family (TC #2.A.75). The PSI-BLAST program groups the LysE family (TC# 2.A.75), the RhtB family and the CadD family (TC #2.A.77) together. These proteins are all of about the same size and apparent topology, further suggesting a common evolutionary origin.
The first two members of the RhtB family to be characterized functionally were the RhtB and RhtC permeases of E. coli (Aleshin et al., 1999; Zakataeva et al., 1999). YfiK of E. coli exports cysteine, O-acetylserine and azaserine (Franke et al., 2003). The YeaS (LeuE) homologue exports leucine and several other neutral, hydrophobic amino acids (Kutukova et al., 2005). Aleshin et al. (1999) present a partial alignment of recognized bacterial and archael members of the RhtB and LysE families, but not the CadD family. Vrljic et al. (1999) present phylogenetic trees for the original three families of the LysE superfamily (LysE, RhtB and CadD).
The transport reaction presumably catalyzed by members of the RhtB family is:
amino acid (in) + nH+ (out) ⇌ amino acid (out) + nH+ (in)
Homoserine/homoserine lactone/β-hydroxynorvaline efflux permease (probable substrate:H+ antiporter), RhtB (YigK). Expression is under the control of the leucine-responsive protein, Lrp, and amino acids in the medium (Kutukova et al. 2005).
RhtB of E. coli (P0AG34)
RhtB family porter of 210 aas and 6 TMSs. Based on homology, may export serine, threonine, homoserine and/or homoserine lactones. Possibly regulated by a LysR transcription factor. The gene maps adjacent to a probable heterodimeric DMT superfamily transporter (2.A.7.21.5).
RhtB family homologue of Klebsiella oxytoca
Amino acid exporter of 210 aas and 6 TMSs, MrsC. Encoded by the mrsC gene in an operon for the biosynthesis of the toxic amino acid analogue, 3-methyarginine, the likely substrate (Braun et al. 2010). The epiphytic Pseudomonas syringae strain which produces and excretes 3-methyarginine does so to compete with pathogenic strains of P. syringae for which 3-methyarginine is toxic.
MrsC of Pseudomonas syringae
Uncharacterized amino acid transporter of 207 aas and 6 TMSs.
UP of Candidatus Wolfebacteria bacterium
LysE superfamily member of 220 aas and 6 TMSs in a 3 + 3 arrangement
LysE member of Lokiarchaeum sp. GC14_75 (from a marine sediment metagenome)
Threonine efflux permease (probable threonine:H+ antiporter), RhtC (YigJ). Expression is under the control of the leucine-responsive protein, Lrp, and amino acids in the medium (Kutukova et al. 2005). Mutation of the rhtC gene can protect cells against toxic levels of threonine (Radi et al. 2022).
RhtC (YigJ) of E. coli (P0AG38)
Probable amino acid exporter, YahN or YhaM, of 233 aas and 6 TMSs. Expression is under the control of the leucine-responsive protein, Lrp, and amino acids in the medium (Kutukova et al. 2005). The yahOM operon is induced by cysteine where the cysteine sensor is the DecR/YbaO protein, and this operon is involved in L-cysteine detoxification (Shimada et al. 2016).
YahN of E. coli (P75693)
O-aetylserine/cysteine/azaserine exporter, EamB or YfiK of 195 aas and 6 TMSs (Franke et al., 2003). High level cell-free expression and specific labeling of EamB of E. coli has been achieved (Klammt et al. 2004).
EamB or YfiK of E. coli
The leucine exporter (LeuE; YeaS) (binds and probably transports L-methionine, L-histidine, L-α-amino-n-butyrate, norleucine, azaleucine, S(2-aminoethyl)-L-cysteine, 3,4-dehydro-DL-proline, L-alanine, and both D- and L-valine) (Kutukova et al., 2005; Hori et al., 2011; Radi et al. 2022). Expression is under the control of the leucine-responsive protein, Lrp, and amino acids in the medium (Kutukova et al. 2005). Mutation of the leuE gene allows tolerance to high levels of histidine, presumably by allowing histidine efflux (Radi et al. 2022).
LeuE of E. coli (P76249)
The RhtB family member, RSP_2720
RSP_2070 of Rhodobacter sphaeroides (Q3J2V9)
Putative amino acid exporter of 208 aas; RhtB family member; similar to 2.A.76.1.5.
RhtB family member of Bacillus subtilis
Putative amino acid efflux porter of 210 aas, YrhP. Most similar to 2.A.76.1.5 of the characterized proteins in TCDB.
YrhP of Bacillus subtilis
Exporter for L-homoserine, L-serine, and L-homoserine lactone as well as the quorum-sensing pimaricin-inducer PI-factor (2,3-diamino-2,3-bis(hydroxymethyl)-1,4-butanediol), PimT (Vicente et al. 2009).
PimT of Streptomyces natalensis
Potential coronamic acid exporter of 236 aas and 5 TMSs. Coronamic acid (CMA), an ethylcyclopropyl amino acid derived from isoleucine, functions as an intermediate in the biosynthesis of coronatine, a chlorosis-inducing phytotoxin produced by Pseudomonas syringae pv. glycinea PG4180. The DNA required for CMA biosynthesisincludes three distinct open reading frames (ORFs) which share a common orientation for transcription. Two ORFs in the operon include CmaT, a thioesterase and CmaU, a potential exporter (Ullrich and Bender 1994).
CmaU of Pseudomonas syringae
CmaU of 203 aas and 5 or 6 TMSs. This protein resembles 2.A.76.2.1; one or the other may be an incomplete or inaccurate sequence.
CmaU of Pseudomonas syringae
CmaU homologue of 203 aas and 6 TMSs.
CmaU of Achromobacter xylosoxidans
Uncharacterized probable amino acid exporter of 230 aas and 6 TMSs.
UP of Croceibacter atlanticus