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Articles by Mengli Cai
Total Records ( 2 ) for Mengli Cai
  Jun Hu , Kaifeng Hu , David C. Williams, Jr. , Michal E. Komlosh , Mengli Cai and G. Marius Clore
  Solution structures of complexes between the isolated A (IIAMan) and B (IIBMan) domains of the cytoplasmic component of the mannose transporter of Escherichia coli have been solved by NMR. The complex of wild-type IIAMan and IIBMan is a mixture of two species comprising a productive, phosphoryl transfer competent complex and a non-productive complex with the two active site histidines, His-10 of IIAMan and His-175 of IIBMan, separated by ∼25Å. Mutation of the active site histidine, His-10, of IIAMan to a glutamate, to mimic phosphorylation, results in the formation of a single productive complex. The apparent equilibrium dissociation constants for the binding of both wild-type and H10E IIAMan to IIBMan are approximately the same (KD ∼ 0.5 mM). The productive complex can readily accommodate a transition state involving a pentacoordinate phosphoryl group with trigonal bipyramidal geometry bonded to the Nε2 atom of His-10 of IIAMan and the Nδ1 atom of His-175 of IIBMan with negligible (<0.2Å) local backbone conformational changes in the immediate vicinity of the active site. The non-productive complex is related to the productive one by a ∼90° rotation and ∼37Å translation of IIBMan relative to IIAMan, leaving the active site His-175 of IIBMan fully exposed to solvent in the non-productive complex. The interaction surface on IIAMan for the non-productive complex comprises a subset of residues used in the productive complex and in both cases involves both subunits of IIAMan. The selection of the productive complex by IIAMan(H10E) can be attributed to neutralization of the positively charged Arg-172 of IIBMan at the center of the interface. The non-productive IIAMan-IIBMan complex may possibly be relevant to subsequent phosphoryl transfer from His-175 of IIBMan to the incoming sugar located on the transmembrane IICMan-IIDMan complex.
  Jeong-Yong Suh , Mengli Cai and G. Marius Clore
  The structural and thermodynamic impact of phosphorylation on the interaction of the N-terminal domain of enzyme I (EIN) and the histidine phosphocarrier protein (HPr), the two common components of all branches of the bacterial phosphotransferase system, have been examined using NMR spectroscopy and isothermal titration calorimetry. His-189 is located at the interface of the α and αβ domains of EIN, resulting in rather widespread chemical shift perturbation upon phosphorylation, in contrast to the highly localized perturbations seen for HPr, where His-15 is fully exposed to solvent. Residual dipolar coupling measurements, however, demonstrate unambiguously that no significant changes in backbone conformation of either protein occur upon phosphorylation: for EIN, the relative orientation of the α and αβ domains remains unchanged; for HPr, the backbone φ/Ψ torsion angles of the active site residues are unperturbed within experimental error. His → Glu/Asp mutations of the active site histidines designed to mimic the phosphorylated states reveal binding equilibria that favor phosphoryl transfer from EIN to HPr. Although binding of phospho-EIN to phospho-HPr is reduced by a factor of ~21 relative to the unphosphorylated complex, residual dipolar coupling measurements reveal that the structures of the unphosphorylated and biphosphorylated complexes are the same. Hence, the phosphorylation states of EIN and HPr shift the binding equilibria predominantly by modulating intermolecular electrostatic interactions without altering either the backbone scaffold or binding interface. This facilitates highly efficient phosphoryl transfer between EIN and HPr, which is estimated to occur at a rate of ~850 s-1 from exchange spectroscopy.
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