Escherichia coli AlkB and its human homologues ABH2 and ABH3 repair DNA/RNA base lesions by using a direct oxidative dealkylation mechanism. ABH2 has the primary role of guarding mammalian genomes against 1-meA damage by repairing this lesion in double-stranded DNA (dsDNA), whereas AlkB and ABH3 preferentially repair single-stranded DNA (ssDNA) lesions and can repair damaged bases in RNA. Here we show the first crystal structures of AlkB-dsDNA and ABH2-dsDNA complexes, stabilized by a chemical cross-linking strategy. This study reveals that AlkB uses an unprecedented base-flipping mechanism to access the damaged base: it squeezes together the two bases flanking the flipped-out one to maintain the base stack, explaining the preference of AlkB for repairing ssDNA lesions over dsDNA ones. In addition, the first crystal structure of ABH2, presented here, provides a structural basis for designing inhibitors of this human DNA repair protein.Cellular DNA is constantly subjected to modifications by environmental and endogenous chemicals, which can result in covalent changes 1,2 . Methylating (or alkylating) agents are a common group of DNA modifiers that introduce damage primarily to the heterocyclic bases of DNA, with mutagenic and/or cytotoxic consequences. Alkylating agents are also widely used in cancer therapy and exert anticancer effects by creating cytotoxic DNA lesions in tumour cells. Many of these alkylation DNA damages are detected and repaired by proteins that are conserved across kingdoms.The E. coli AlkB protein is a direct dealkylation DNA repair protein 3-5 . It uses a mononuclear iron(II) site and cofactors 2-ketoglutarate (2KG) and dioxygen to perform an unprecedented oxidative demethylation of DNA base lesions 1-meA, 3-meC, 1-meG and 3-meT ( Supplementary Fig. 2) 6-11 . AlkB also removes etheno DNA lesions by using a similar oxidation mechanism 12,13 . There are nine potential human homologues of AlkB. Two of Correspondence and requests for materials should be addressed to C.H. (chuanhe@uchicago.edu). * These authors contributed equally to this work. Reprints and permissions information is available at www.nature.com/reprints.
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Cross-linking to stabilize protein-DNA complexesWe report here the first crystal structures of AlkB-dsDNA and ABH2-dsDNA complexes. The AlkB family proteins bind DNA weakly 21 and form labile complexes with damagecontaining DNA 22 , which makes crystallization of their protein-DNA complexes challenging.To overcome this difficulty we used chemical cross-linking methods 23,24 ; initially using an active site disulphide cross-linking strategy that we developed previously (Fig. 1a) 25,26 . Baserepair proteins flip damaged bases and insert them into the active site for processing. Therefore, we reasoned, a cysteine residue engineered into the active site of AlkB may form a disulphide cross-link, at equilibrium, with a disulphide-modified cytosine (C* in a C*:A base pair) flipped into the active site of the repair protein ( Fig. 1a) 27 ....