The Zβ domain of human DAI binds to Z-DNA via a novel B-Z transition pathway

The human DNA-dependent activator of IFN-regulatory factor (DAI) protein, which activates the innate immune response in response to DNA, contains two tandem Z-DNA binding domains (Zα and Zβ) at the NH₂ terminus. The hZβ_DAI structure is similar to other Z-DNA binding proteins, although it demonstrates an unusual Z-DNA recognition. We performed NMR experiments on complexes of hZβ_DAI with DNA duplex, d(CGCGCG)₂, at a variety of protein-to-DNA molar ratios. The results suggest that hZβ_DAI binds to Z-DNA via an active-di B-Z transition mechanism, where two hZβ_DAI proteins bind to B-DNA to form the hZβ_DAI-B-DNA complex; the B-DNA is subsequently converted to left-handed Z-DNA. This novel mechanism of DNA binding and B-Z conversion is distinct from Z-DNA binding of the human ADAR1 protein.