Structure of a zinc-finger antiviral protein in complex with RNA reveals mechanism for selective targeting of CG-rich viral sequences

Infection of animal cells by many viruses is detected and countered by a variety of means, including recognition of non-self nucleic acids. The zinc-finger antiviral protein (ZAP) depletes cytoplasmic RNA that is recognized as foreign in mammalian cells by virtue of its elevated CG dinucleotide content compared to endogenous mRNAs. Here, we determined a crystal structure of a protein-RNA complex containing the N-terminal, four-zinc finger human (h) ZAP RNA binding domain (RBD), and a CG dinucleotide-containing RNA target. The structure reveals in molecular detail how hZAP is able to bind selectively to CG-rich RNA. Specifically, the four zinc fingers create a basic patch on the hZAP RBD surface. The highly basic second zinc finger contains a pocket that selectively accommodates CG dinucleotide bases. Structure guided mutagenesis, crosslinking-immunoprecipitation-sequencing assays, and RNA affinity assays show that the structurally defined CG-binding pocket is not required for RNA binding per se in human cells. However, the pocket is a crucial determinant of high affinity specific binding to CG-dinucleotide-containing RNA. Moreover, variations in the RNA binding specificity a panel of CG-binding pocket mutants quantitatively predict their selective antiviral activity against a CG-enriched HIV-1 strain. Overall, the hZAP RBD:RNA structure provides an atomic-level explanation for how ZAP selectively targets foreign, CG-rich RNA. CLIP-seq data sets from 293T cells