Active and alkylated human AGT structures: a novel zinc site, inhibitor and extrahelical base binding

EMBO J. 2000 Apr 3;19(7):1719-30. doi: 10.1093/emboj/19.7.1719.

Abstract

Human O(6)-alkylguanine-DNA alkyltransferase (AGT), which directly reverses endogenous alkylation at the O(6)-position of guanine, confers resistance to alkylation chemotherapies and is therefore an active anticancer drug target. Crystal structures of active human AGT and its biologically and therapeutically relevant methylated and benzylated product complexes reveal an unexpected zinc-stabilized helical bridge joining a two-domain alpha/beta structure. An asparagine hinge couples the active site motif to a helix-turn-helix (HTH) motif implicated in DNA binding. The reactive cysteine environment, its position within a groove adjacent to the alkyl-binding cavity and mutational analyses characterize DNA-damage recognition and inhibitor specificity, support a structure-based dealkylation mechanism and suggest a molecular basis for destabilization of the alkylated protein. These results support damaged nucleotide flipping facilitated by an arginine finger within the HTH motif to stabilize the extrahelical O(6)-alkylguanine without the protein conformational change originally proposed from the empty Ada structure. Cysteine alkylation sterically shifts the HTH recognition helix to evidently mechanistically couple release of repaired DNA to an opening of the protein fold to promote the biological turnover of the alkylated protein.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Alkylation
  • Amino Acid Sequence
  • Base Sequence
  • Binding Sites / genetics
  • Catalytic Domain / genetics
  • Crystallography, X-Ray
  • DNA / genetics
  • DNA / metabolism
  • Enzyme Activation
  • Enzyme Inhibitors / pharmacology
  • Helix-Loop-Helix Motifs
  • Humans
  • In Vitro Techniques
  • Models, Molecular
  • Molecular Sequence Data
  • Mutagenesis, Site-Directed
  • O(6)-Methylguanine-DNA Methyltransferase / chemistry*
  • O(6)-Methylguanine-DNA Methyltransferase / genetics
  • O(6)-Methylguanine-DNA Methyltransferase / metabolism*
  • Protein Conformation
  • Recombinant Proteins / chemistry
  • Recombinant Proteins / genetics
  • Recombinant Proteins / metabolism
  • Sequence Homology, Amino Acid
  • Substrate Specificity
  • Zinc / chemistry

Substances

  • Enzyme Inhibitors
  • Recombinant Proteins
  • DNA
  • O(6)-Methylguanine-DNA Methyltransferase
  • Zinc