2.A.75 The L-Lysine Exporter (LysE) Family

Two members of the LysE family (LysE of Corynebacterium glutamicum and ArgO of E. coli) have been functionally characterized, but functionally uncharacterized homologues are encoded within the genomes of many bacteria including Mycobacterium tuberculosis, Bacillus subtilis, Aeromonas salmonicida, Helicobacter pylori, Vibrio cholerae and Yersinia pestis. Thus, LysE family members are found widely distributed in Gram-negative and Gram-positive bacteria. These proteins are 190-240 amino acyl residues in length and possess six hydrophobic regions. PhoA fusion analyses of LysE of C. glutamicum provided evidence for a 5 transmembrane α-helical spanner (TMS) typology with the N-terminus inside and the C-terminus outside (Vrljic et al., 1999). However, some evidence suggests a 6 TMS topology (R. Kramer, personal communication).

LysE appears to catalyze unidirectional efflux of L-lysine (and other basic amino acids such as L-arginine), and it provides the sole route for L-lysine excretion. The energy source is believed to be the proton motive force (H⁺ antiport). The E. coli ArgO homologue effluxes arginine and possibly lysine and canavanine as well (Nandineni and Gowrishankar, 2004).

Early studies showed that the LysE family is related to the RhtB family (TC #2.A.76) as well as the CadD family (TC #2.A.77) based both on the similar sizes and topologies of their members and on PSI-BLAST results (Vrljic et al., 1999). Thus, three families comprise the LysE superfamily, the members of which are restricted to bacteria and archaea.

The generalized transport reaction for LysE is:

Lysine (in) ⁺ [nH⁺ (out) or nOH^- (in)] Lysine (out) ⁺ [nH⁺ (in) or nOH^- (out)]