Trans-kingdom mimicry underlies ribosome customization by a poxvirus kinase

Ribosomes have the capacity to selectively control translation through changes in their composition that enable recognition of specific RNA elements(1). However, beyond differential subunit expression during development(2,3), evidence for regulated ribosome specification within individual cells has remained elusive(1). Here, we report that a poxvirus kinase phosphorylates serine/threonine residues in the small ribosomal subunit protein, Receptor for Activated C Kinase (RACK1) that are not phosphorylated in uninfected cells or cells infected by other viruses. These modified residues cluster in an extended loop in RACK1, phosphorylation of which selects for translation of viral or reporter mRNAs whose 5’ untranslated regions (UTRs) contain adenosine repeats, so-called polyA-leaders. Structural and phylogenetic analysis revealed that although RACK1 is highly conserved, this loop is variable and contains negatively charged amino acids in plants, where these leaders act as translational enhancers for poorly understood reasons. Phosphomimetics and inter-species chimeras demonstrated that negative charge in the RACK1 loop dictates ribosome selectivity towards viral RNAs. By converting human RACK1 to a charged, plant-like state, poxviruses remodel host ribosomes so that adenosine repeats erroneously generated by slippage of the viral RNA polymerase(4) confer a translational advantage. Our findings uncover ribosome customization through a novel trans-kingdom mimicry and the mechanics of species-specific leader activity that underlie the enigmatic poxvirus polyA-leaders(4).