The ubiquitous ThrE family of putative transmembrane amino acid efflux transporters.

We here report sequence analyses of a newly described family of putative amino acid exporters, the ThrE family. Homologues were identified in select bacteria, archaea and eukaryotes, but only in the fungal kingdom of eukaryotes. These proteins can exist either as single polypeptide chains or as pairs of polypeptide chains. Computational evidence suggests that these proteins exhibit 10 transmembrane α-helical segments (TMSs), having arisen from a five TMS precursor by an early intragenic duplication event. The phylogenetic tree of the ThrE family reveals that most proteins cluster according to organismal phylogeny with only a few exceptions, suggesting that the former proteins are orthologues. All family members exhibit hydrophilic N-terminal (and occasional C-terminal) extensions that show limited sequence similarity with a domain of unknown function found in many peptidases and proteases. The significance of these observations is discussed.