title: The "Transport Specificity Ratio": a structure-function tool to search the protein fold for loci that control transition state stability in membrane transport catalysis.
abstract: BACKGROUND
  In establishing structure-function relationships for membrane transport proteins, the interpretation of phenotypic changes can be problematic, owing to uncertainties in protein expression levels, sub-cellular localization, and protein-folding fidelity. A dual-label competitive transport assay called "Transport Specificity Ratio" (TSR) analysis has been developed that is simple to perform, and circumvents the "expression problem," providing a reliable TSR phenotype (a constant) for comparison to other transporters.
  
  
  RESULTS
  Using the Escherichia coli GABA (4-aminobutyrate) permease (GabP) as a model carrier, it is demonstrated that the TSR phenotype is largely independent of assay conditions, exhibiting: (i) indifference to the particular substrate concentrations used, (ii) indifference to extreme changes (40-fold) in transporter expression level, and within broad limits (iii) indifference to assay duration. The theoretical underpinnings of TSR analysis predict all of the above observations, supporting that TSR has (i) applicability in the analysis of membrane transport, and (ii) particular utility in the face of incomplete information on protein expression levels and initial reaction rate intervals (e.g., in high-throughput screening situations). The TSR was used to identify gab permease (GabP) variants that exhibit relative changes in catalytic specificity (kcat/Km) for [14C]GABA (4-aminobutyrate) versus [3H]NA (nipecotic acid).
  
  
  CONCLUSIONS
  The TSR phenotype is an easily measured constant that reflects innate molecular properties of the transition state, and provides a reliable index of the difference in catalytic specificity that a carrier exhibits toward a particular pair of substrates. A change in the TSR phenotype, called a Delta(TSR), represents a specificity shift attributable to underlying changes in the intrinsic substrate binding energy (DeltaGb) that translocation catalysts rely upon to decrease activation energy (Delta G(T)(++). TSR analysis is therefore a structure-function tool that enables parsimonious scanning for positions in the protein fold that couple to the transition state, creating stability and thereby serving as functional determinants of catalytic power (efficiency, or specificity).
authors.affiliation: Integrative Biosciences, Oregon Health & Science University, Portland, Oregon 97239-3097, USA. kingst@ohsu.edu
authors.forename: Steven C
authors.initials: SC
authors.lastname: King
mesh_terms: D001667:Binding, Competitive / Q000502:physiology; D002384:Catalysis; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D050485:GABA Plasma Membrane Transport Proteins; D015964:Gene Expression Regulation, Bacterial / Q000502:physiology; D015971:Gene Expression Regulation, Enzymologic / Q000502:physiology; D014644:Genetic Variation / Q000502:physiology; D008958:Models, Molecular; D009557:Nipecotic Acids / Q000378:metabolism; D027361:Organic Anion Transporters / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D010641:Phenotype; D017510:Protein Folding; D021281:Quantitative Structure-Activity Relationship; D013379:Substrate Specificity / Q000502:physiology; D005680:gamma-Aminobutyric Acid / Q000378:metabolism
chemical_list: D029968:Escherichia coli Proteins; D050485:GABA Plasma Membrane Transport Proteins; D009557:Nipecotic Acids; D027361:Organic Anion Transporters; C513601:gabP protein, E coli; C030278:nipecotic acid; D005680:gamma-Aminobutyric Acid