Ligand recognition properties of the Escherichia coli 4-aminobutyrate transporter encoded by gabP: Specificity of Gab permease for heterocyclic inhibitors

4-aminobutyrate metabolism in Escherichia coli begins with transport across the cytoplasmic membrane via the GabP, which is encoded by gabP. Although GabP is specific and exhibits poor affinity for many cellular constituents such as the α-amino acids, the range of compounds recognized with high affinity has yet to be investigated. In order to address this gap in knowledge, we developed a gabP-negative host strain, which permits evaluation of test compounds for inhibitory effects on cloned GabP (expression inducible by isopropyl-1-thio-β-D-galactopyranoside). Using this inducible expression system, three structurally distinct categories of high affinity transport inhibitor were identified. The structural dissimilarity of these inhibitors significantly alters our view of ligand recognition by GabP. Any complete model must now account for the observation that inhibition of 4-aminobutyrate transport can be mediated either (i) by open chain analogs of 4-aminobutyrate, (ii) by cyclic amino acid analogs, or (iii) by planar heterocyclic compounds lacking a carboxyl group. Such results do not support a previously sustainable view of GabP that features a restrictive ligand recognition domain, unable to accommodate structures that differ very much from the native substrate, 4-aminobutyrate.