Damage detection and base flipping in direct DNA alkylation repair.

THE FOREIGN LESION: The mechanistic questions for DNA base damage detection by repair proteins are discussed in this Minireview. Repair proteins could either probe and locate a weakened base pair that results from base damage, or passively capture an extrahelical base lesion in the first step of damage searching on double-stranded DNA. How some repair proteins, such as AGT (see figure), locate base lesions in DNA is still not fully understood.To remove a few damaged bases efficiently from the context of the entire genome, the DNA base repair proteins rely on remarkably specific detection mechanisms to locate base lesions. This efficient molecular recognition event inside cells has been extensively studied with various structural and biochemical tools. These studies suggest that DNA base damage can be located by repair proteins by using two mechanisms: a repair protein can probe and detect a weakened base pair that results from mutagenic or cytotoxic base damage; alternatively, a protein can passively capture and stabilize an extrahelical base lesion. Our chemical and structural studies on the direct DNA repair proteins hAGT, C-Ada and ABH2 suggest that these proteins search for weakened base pairs in their first step of damage searching. We have also discovered a very unique base-flipping mechanism used by the DNA repair protein AlkB. This protein distorts DNA and favors single stranded DNA (ssDNA) substrates over double-stranded (dsDNA) ones. Potentially, it locates base lesions in dsDNA by imposing a constraint that targets less rigid regions of the duplex DNA. The exact mechanism of how AlkB and related proteins search for damage in ssDNA and dsDNA still awaits further studies.