2.A.86 The Autoinducer-2 Exporter (AI-2E) Family (formerly the PerM Family, TC #9.B.22)

  The AI-2E family (UPF0118) is a large family of prokaryotic proteins derived from a variety of bacteria and archaea. Those examined are about 350 residues in length, and the couple that have been examined exhibit 7 putative TMSs (Rettner and Saier, 2010). E. coli, B. subtilis and several other prokaryotes have multiple paralogues encoded within their genomes. Herzberg et al. (2006) have presented strong evidence for a role of a AI-2E family homologue, YdgG (renamed TqsA) is an exporter of the E. coli autoinducer-2 (AI-2) (Camilli and Bassler, 2006; Chen et al., 2002). AT-2 is a proposed signalling molecule for interspecies communication in bacteria. It is a furanosyl borate diester (Chen et al., 2002). It is induced in Bacillus subtilis by exposure to rice seedlings (Xie et al. 2015).  AI-2, a universal molecule for both intra- and inter-species communication, is involved in the regulation of biofilm formation, virulence, motility, chemotaxis, and antibiotic resistance. (Khera et al. 2022).
   More recently, it has been reported that this family includes a member of the UPF0118 family (which was the former designation for the AI-2E family), and this transmembrane protein with 7 TMSs, exhibits reversible pH-dependent Na⁺ or Li⁺/H⁺ antiport activity. Phylogenetic analyses were reported (Dong et al. 2017).  Thus, it appears that different members of the family may have very different transport functions. Cryo-EM structures of two pentameric autoinducer-2 exporter from E. coli (TqsA (TC# 2.A.86.1.4) and YdiK (TC# 2.A.86.2.1) revealed the probable transport mechanism (Khera et al. 2022). Each of the 5 subunits is believed to be a functional unit, and an elevator-type mechanism has been suggested.

The transport reactions catalyzed by membeers of the AI-2E family are:

AI-2 (in) ⇌ AI-2 (out)

Na⁺ or Li⁺ (out) + H⁺ (in) → Na⁺ or Li⁺ (in) + H⁺ (out)