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 hZβDAI structure is similar to other Z-DNA binding proteins, although it demonstrates an unusual Z-DNA recognition. We performed NMR experiments on complexes of hZβDAI with DNA duplex, d(CGCGCG)2, at a variety of protein-to-DNA molar ratios. The results suggest that hZβDAI binds to Z-DNA via an active-di B–Z transition mechanism, where two hZβDAI proteins bind to B-DNA to form the hZβDAI–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.