1.A.112.2.11
PaeA, YtfL, UPF0053 inner membrane protein, Duf21 domain containing protein, HlyC/CorC family transporter, hemolysin homolog of 447 aas and 4 N-terminal TMSs (residues 1 - 200) followed by a large hydrophilic domain (cystathionine beta-synthase, CBS, residues 201 - 447), possibly with a single TMS at about residue 320. It transports cadaverine and putrescine.  In fact, Salmonella, Klebsiella pneumoniae (TC# 1.A.112.1.12) and E. coli synthesize, import, and export cadaverine, putrescine, and spermidine to maintain physiological levels of polyamines and provide pH homeostasis. Both low and high intracellular levels of polyamines confer pleiotropic phenotypes or lethality. Iwadate et al. 2021 demonstrated that PaeA (YtfL) is required for reducing cytoplasmic cadaverine and putrescine concentrations. PaeA is involved in stationary phase survival when cells are grown in acidic medium in which they produce cadaverine. The paeA mutant is sensitive to putrescine, but not spermidine or spermine. Sensitivity to external cadaverine in stationary phase is only observed at pH > 8, suggesting that the polyamines need to be deprotonated to passively diffuse into the cell. In the absence of PaeA, intracellular polyamine levels increase and the cells lose viability. Ectopic expression of the known cadaverine exporter, CadB, in stationary phase partially suppresses the paeA mutant phenotype, and overexpression of paeA in exponential phase partially complements a cadB mutant grown in acidic medium. Thus, PaeA is a cadaverine/putrescine exporter, reducing potentially toxic levels under certain stress conditions (Iwadate et al. 2021).