2.A.104 The L-Alanine Exporter (AlaE) Family
The AlaE family is the same as the DUF1144 Family. A mutant that is hypersensitive to L-alanyl-L-alanine from a non-L-alanine-metabolizing E. coli strain lacked an inducible L-alanine export system and accumulates intracellular L-alanine with a reduction in the L-alanine export rate. When the mutant was used to clone genes that complement the dipeptide-hypersensitive phenotype, two uncharacterized genes, ygaW and ytfF, and two characterized genes, yddG and yeaS, were identified (Hori et al., 2011). Overexpression of each gene in the mutant resulted in a decrease in the intracellular L-alanine level and enhancement of the L-alanine export rate in the presence of the dipeptide, suggesting that their products function as exporters of L-alanine. Since ygaW exhibited the most striking impact on both the intra- and the extracellular L-alanine levels among the four genes identified, Hori et al. (2011) disrupted the ygaW gene in the non-L-alanine-metabolizing strain. The resulting isogenic mutant showed the same intra- and extracellular L-alanine levels as observed in the dipeptide-hypersensitive mutant obtained by chemical mutagenesis. When each gene was overexpressed in the wild-type strain, which does not intrinsically excrete alanine, only the ygaW gene conferred on the cells the ability to excrete alanine. In addition, expression of the ygaW gene was induced in the presence of the dipeptide. Thus, ygaW is likely to be the gene encoding the physiologically most relevant exporter for L-alanine in E. coli. AlaE forms homo-oligomers, and the GxxxG motif in the TMS4 region plays an essential role in AlaE activity but not in AlaE oligomer formation (Ihara et al. 2022).