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 determined the structure of QatC bound to the ATP analog AMPPNP and performed structure-guided functional assays. We further found that QatB and QatC form a stable heterodimer and solved the structure of the QatB-QatC complex. In addition to unveiling the structure of QatB, structural analysis suggested 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.