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)