title: Structure of a variant of lac repressor with increased thermostability and decreased affinity for operator.
abstract: A single amino acid substitution, K84L, in the Escherichia coli lac repressor produces a protein that has substantially increased stability compared to wild-type. However, despite the increased stability, this altered tetrameric repressor has a tenfold reduced affinity for operator and greatly decreased rate-constants of inducer binding as well as a reduced phenotypic response to inducer in vivo. To understand the dramatic increase in stability and altered functional properties, we have determined the X-ray crystal structures of a dimeric repressor with and without the K84L substitution at resolutions of 1.7 and 3.0 A, respectively. In the wild-type dimer, K84-11, Lys84 forms electrostatic interactions at the monomer-monomer interface and is partially exposed to solvent. In the K84L-11 substituted protein there is reorientation of the N-subdomains, which allows the leucine to become deeply buried at the monomer-monomer interface. This reorientation of the N-subdomains, in turn, results in an alteration of hydrogen bonding, ion pairing, and van der Waals interactions at the monomer-monomer interface. The lysine residue at position 84 appears to exert its key effects by destabilizing the "optimal" conformation of the repressor, effectively loosening the dimer interface and allowing the repressor to adopt the conformations necessary to function as a molecular switch.
authors.affiliation: The Johnson Foundation and Department of Biochemistry and Biophysics, University of Pennsylvania School of Medicine, 37th and Hamilton Walk, Philadelphia, PA 19102-6059, USA.
authors.forename: C E
authors.initials: CE
authors.lastname: Bell
authors.forename: J
authors.initials: J
authors.lastname: Barry
authors.forename: K S
authors.initials: KS
authors.lastname: Matthews
authors.forename: M
authors.initials: M
authors.lastname: Lewis
mesh_terms: D019943:Amino Acid Substitution / Q000235:genetics; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D018360:Crystallography, X-Ray; D004269:DNA, Bacterial / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D019281:Dimerization; D004926:Escherichia coli / Q000737:chemistry*; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D014644:Genetic Variation / Q000235:genetics; D006860:Hydrogen Bonding; D007700:Kinetics; D056968:Lac Repressors; D008958:Models, Molecular; D009154:Mutation / Q000235:genetics*; D009875:Operator Regions, Genetic / Q000235:genetics*; D011487:Protein Conformation; D012097:Repressor Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D012997:Solvents / Q000378:metabolism; D055672:Static Electricity; D013816:Thermodynamics
chemical_list: D001426:Bacterial Proteins; D004269:DNA, Bacterial; D029968:Escherichia coli Proteins; D056968:Lac Repressors; C539064:LacI protein, E coli; D012097:Repressor Proteins; D012997:Solvents