Animal and bacterial cells defend against viral infection by rapidly activating antiviral restriction factors. In human cells, antiviral immunity is initiated by interferon signaling that results in expression of hundreds of interferon-stimulated genes (ISGs). Complex regulatory networks and co-evolution of viral evasion strategies complicate analysis of immune proteins under native conditions and the function of most individual ISGs remains unknown. Here we discover that heterologous expression of ISGs in bacteria is sufficient to protect against infection by diverse bacteriophages demonstrating properties of antiviral restriction preserved across billions of years of viral evolution. A screen of 306 human ISGs against 11 E. coli phages reveals that ISGs can restrict phage replication with potency equal to endogenous bacterial defense systems. We select for phage mutants that escape ISG restriction and identify the phage DNA primase-helicase complex as a target of human SPSB1. A 2.0 angstrom crystal structure of the SPSB1-primase complex uncovers a recognition mechanism of foreign DxNxN protein motifs found in many animal and bacterial viral replication proteins. We show that SPSB1 in human cells recognizes and induces degradation of protein targets containing this motif from norovirus and poxvirus pathogens. Our results establish a cross-kingdom approach to studying immune function and reveal that human ISGs target features of viral replication common across kingdoms of life.
Fernandez, S. G., Hutchinson, J. E., Tan, J. M., Yamaguchi, S., Roffler, A. A., Schmidt, A. G., Kranzusch, P. J.
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