Zα and Zβ domains of ADAR1 and ZBP1 bind G-quadruplexes with nanomolar affinities, establishing Zβ as a G-quadruplex-specific domain.

While it is well established that the Zα domains of ADAR1 and ZBP1 proteins bind Z-form-prone nucleic acids (Z-NAs), it has also been shown that the Zα domain of ADAR1 binds DNA G-quadruplexes (GQs). However, no binding partner of the structurally homologous Zβ domain of ADAR1 has been identified to date. Based on AlphaFold and molecular dynamics simulations, it has recently been suggested that Zβ of ADAR1 targets its substrate by recognizing GQs. Here, we provide the first experimental evidence for Zβ binding to select GQ RNA and DNA in vitro, with structural specificity and nanomolar affinity. We also demonstrate that the Zα domains of ZBP1 and ADAR1 bind to both DNA and RNA GQs with similar affinity. These findings extend the range of potential functional roles for these proteins and open new hypotheses for testing in cells.