Structure of the Helicase Domain of DNA Polymerase Theta Reveals a Possible Role in the Microhomology-Mediated End-Joining Pathway

DNA polymerase theta (Pol theta) has been identified as a crucial alternative non-homologous end-joining factor in mammalian cells. Pol theta is upregulated in a range of cancer cell types defective in homologous recombination, and knockdown has been shown to inhibit cell survival in a subset of these, making it an attractive target for cancer treatment. We present crystal structures of the helicase domain of human Pol theta in the presence and absence of bound nucleotides, and a characterization of its DNA-binding and DNA-stimulated ATPase activities. Comparisons with related helicases from the Hel308 family identify several unique features. Pol theta exists as a tetramer both in the crystals and in solution. We propose a model for DNA binding to the Pol theta helicase domain in the context of the Pol theta tetramer, which suggests a role for the helicase domain in strand annealing of DNA templates for subsequent processing by the polymerase domain.