Structural investigation of QatB and QatC proteins in QatABCD anti-phage defense

QatABCD is a widespread anti-phage defense system in prokaryotes comprising four protein components. QatC, a signature component, is homologous to QueC, an enzyme involved in nucleobase modification during queuosine biosynthesis. QatA and QatD are predicted to function as an ATPase and a nuclease, respectively, while QatB lacks identifiable sequence motifs. Here, we report the structural and functional characterization of QatB and QatC. We determine the structure of QatC bound to the ATP analog AMPPNP and perform structure-guided functional assays. We further find that QatB and QatC form a stable heterodimer and solve the structure of the QatB-QatC complex. In addition to determining the structure of QatB, structural analysis suggests that it may serve as a substrate of QatC, implicating a potential regulatory mechanism. These findings provide structural and functional insights into QatB and QatC, laying a foundation for understanding the molecular mechanism of the QatABCD system in phage defense.