RESUMEN
Amino acid selection by aminoacyl-tRNA synthetases requires efficient mechanisms to avoid incorrect charging of the cognate tRNAs. A proofreading mechanism prevents Escherichia coli methionyl-tRNA synthetase (EcMet-RS) from activating in vivo L-homocysteine, a natural competitor of L-methionine recognised by the enzyme. The crystal structure of the complex between EcMet-RS and L-methionine solved at 1.8 A resolution exhibits some conspicuous differences with the recently published free enzyme structure. Thus, the methionine delta-sulphur atom replaces a water molecule H-bonded to Leu13N and Tyr260O(eta) in the free enzyme. Rearrangements of aromatic residues enable the protein to form a hydrophobic pocket around the ligand side-chain. The subsequent formation of an extended water molecule network contributes to relative displacements, up to 3 A, of several domains of the protein. The structure of this complex supports a plausible mechanism for the selection of L-methionine versus L-homocysteine and suggests the possibility of information transfer between the different functional domains of the enzyme.
Asunto(s)
Escherichia coli/enzimología , Metionina-ARNt Ligasa/química , Metionina-ARNt Ligasa/metabolismo , Metionina/metabolismo , Regulación Alostérica , Sitio Alostérico , Secuencia de Aminoácidos , Unión Competitiva , Cristalización , Cristalografía por Rayos X , Homocisteína/metabolismo , Enlace de Hidrógeno , Metionina/química , Modelos Moleculares , Datos de Secuencia Molecular , Estructura Secundaria de Proteína , Estructura Terciaria de Proteína , Alineación de Secuencia , Especificidad por Sustrato , Agua/química , Agua/metabolismoRESUMEN
The homodimeric form alpha(2) of the Escherichia coli DNA-binding protein HU was crystallized by the hanging-drop vapour-diffusion method using PEG 4000 as a precipitant. The crystals belong to space group I222, with unit-cell parameters a = 31.09, b = 55.34, c = 117. 63 A, and contain one monomer per asymmetric unit. A full diffraction data set was collected to 2.3 A resolution on a conventional X-ray source. The molecular-replacement method, using the HU crystallographic model from Bacillus stearothermophilus as a starting point, gave a reliable solution for the rotation and translation functions.
Asunto(s)
Proteínas Bacterianas/química , Proteínas de Unión al ADN/química , Cristalización , Dimerización , Electroforesis en Gel de Poliacrilamida , Escherichia coli , Geobacillus stearothermophilus , Difracción de Rayos XRESUMEN
The binding of Escherichia coli and Lactococcus lactis Fapy-DNA glyosylase (Fpg) proteins to DNA containing either cyclic or non-cyclic abasic (AP) site analogs was investigated by electrophoretic mobility shift assay (EMSA) and by footprinting experiments. We showed that the reduced AP site is the best substrate analog for the E.coli and L.lactis enzymes ( K Dapp = 0.26 and 0.5 nM, respectively) as compared with the other analogs tested in this study ( K Dapp >2.8 nM). The 1,3-propanediol (Pr) residue-containing DNA seems to be the minimal AP site structure allowing a Fpg specific DNA binding, since the ethyleneglycol residue is not specifically bound by these enzymes. The newly described cyclopentanol residue is better recognized than tetrahydrofuran (for the E.coli Fpg, K Dapp = 2.9 and 25 nM, respectively). These results suggest that the hemiacetal form of the AP site is negatively discriminated by the Fpg protein suggesting a hydrogen bond between the C4'-hydroxyl group of the sugar and a Fpg residue. High-resolution hydroxyl radical footprinting using a duplex containing Pr shows that Fpg binds to six nucleotides on the strand containing the AP site and only the base opposite the lesion on the undamaged complementary strand. This comparative study provides new information about the molecular mechanism involved in the Fpg AP lyase activity.