Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UMLS:C0033036 (APC)
 10,214 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The Escherichia coli GABA (gamma-aminobutyric acid) permease GabP is a prototypical APC (amine/polyamine/choline) super-family transporter that has a CAR (consensus amphipathic region) containing multiple specificity determinants, ostensibly organized on two helical surfaces, one hydrophobic [SHS (sensitive hydrophobic surface)] and the other hydrophilic [SPS (sensitive polar surface)]. To gauge the functional effects of placing alanine insertions at close intervals across the entire GabP CAR, 64 insertion variants were constructed. Insertions, particularly those in the SHS and the SPS, were highly detrimental to steady-state [(3)H]GABA accumulation. TSR (transport specificity ratio) analysis, employing [(3)H]nipecotic acid and [(14)C]GABA, showed that certain alanine insertions were associated with a specificity shift (i.e. a change in k (cat)/ K (m)). An insertion (INS Ala-269) located N-terminal to the SHS increased specificity for [(3)H]nipecotic acid relative to [(14)C]GABA, whereas an insertion (INS Ala-321) located C-terminal to the SPS had the opposite effect. Overall, the results are consistent with a working hypothesis that the GabP CAR contains extensive functional surfaces that may be manipulated by insertion mutagenesis to alter the specificity ( k (cat)/ K (m)) phenotype. The thermodynamic basis of TSR analysis provides generality, suggesting that amino acid insertions could affect specificity in many other transporters, particularly those such as the E. coli phenylalanine permease PheP [Pi, Chow and Pittard (2002) J. Bacteriol. 184, 5842-5847] that have a functionally significant CAR-like domain.
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PMID:Induction of substrate specificity shifts by placement of alanine insertions within the consensus amphipathic region of the Escherichia coli GABA (gamma-aminobutyric acid) transporter encoded by gabP. 1295 23

The Escherichia coli GABA (gamma-aminobutyric acid) permease, GabP, and other members of the APC (amine/polyamine/choline) transporter superfamily share a CAR (consensus amphipathic region) that probably contributes to solute translocation. If true, then the CAR should contain structural features that act as determinants of substrate specificity ( k (cat)/ K (m)). In order to address this question, we have developed a novel, expression-independent TSR (transport specificity ratio) analysis, and applied it to a series of 69 cysteine-scanning (single-cysteine) variants. The results indicate that GabP has multiple specificity determinants (i.e. residues at which an amino acid substitution substantially perturbs the TSR). Specificity determinants were found: (i) on a hydrophobic surface of the CAR (from Leu-267 to Ala-285), (ii) on a hydrophilic surface of the CAR (from Ser-299 to Arg-318), and (iii) in a cytoplasmic loop (His-233) between transmembrane segments 6 and 7. Overall, these observations show that (i) structural features within the CAR have a role in substrate discrimination (as might be anticipated for a transport conduit) and, interestingly, (ii) the substrate discrimination task is shared among specificity determinants that appear too widely dispersed across the GabP molecule to be in simultaneous contact with the substrates. We conclude that GabP exhibits behaviour consistent with a broadly applicable specificity delocalization principle, which is demonstrated to follow naturally from the classical notion that translocation occurs synchronously with conformational transitions that change the chemical potential of the bound ligand [Tanford (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 2882-2884].
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PMID:Use of the transport specificity ratio and cysteine-scanning mutagenesis to detect multiple substrate specificity determinants in the consensus amphipathic region of the Escherichia coli GABA (gamma-aminobutyric acid) transporter encoded by gabP. 1295 24