ThrE_2 is a family of membrane proteins involved in the export of threonine and serine. L-threonine, L-serine are both substrates for the exporter. The exporter exhibits nine-ten predicted transmembrane-spanning helices with long charged C and N termini and an amphipathic helix present within the N terminus [1]. L-Threonine can be made by the amino acid-producing bacterium Corynebacterium glutamicum, but the potential for amino acid formation can be considerably improved by reducing its intracellular degradation into glycine and increasing its export by this exporter [2]. Members of the family are found in Bacteria, Archaea, and the fungal kingdoms, and the family can exist either as a single long polypeptide chain or as two short polypeptides [2]. All family members show an extended hydrophilic N-terminal domain with weak sequence similarity to portions of hydrolases (proteases, peptidases, and glycosidases); this suggests that since this region is cytoplasmic to the membrane it may be generating the transport substrate, so may imply that threonine may not be the primary substrate and the ThrE has a subsidiary function [3].

1: L-threonine export: use of peptides to identify a new translocator from Corynebacterium glutamicum. Simic P, Sahm H, Eggeling L; J Bacteriol. 2001;183:5317-5324. PMID:11514515

2: Identification of glyA (encoding serine hydroxymethyltransferase) and its use together with the exporter ThrE to increase L-threonine accumulation by Corynebacterium glutamicum. Simic P, Willuhn J, Sahm H, Eggeling L; Appl Environ Microbiol. 2002;68:3321-3327. PMID:12089010

3: New ubiquitous translocators: amino acid export by Corynebacterium glutamicum and Escherichia coli. Eggeling L, Sahm H; Arch Microbiol. 2003;180:155-160. PMID:12879215