Human host-range restriction of the vaccinia virus C7/K1 double deletion mutant is mediated by an atypical mode of translation inhibition.

Replication of vaccinia virus in human cells depends on the viral C7 or K1 protein. A previous human genome-wide siRNA screen with a C7/K1 double deletion mutant revealed SAMD9 as a principal host-range restriction factor plus additional candidates including WDR6 and FTSJ1. To compare their abilities to restrict replication, the cellular genes were individually inactivated by CRISPR/Cas9 mutagenesis. The C7/K1 deletion mutant exhibited enhanced replication in each knock-out (KO) cell line but reached wild-type levels only in SAMD9 KO cells. SAMD9 was not depleted in either WDR6 or FTSJ1 KO cells, suggesting less efficient alternative rescue mechanisms. Using the SAMD9 KO cells as controls, we verified a specific block in host and viral intermediate and late protein synthesis in HeLa cells and demonstrated that the inhibition could be triggered by events preceding viral DNA replication. Inhibition of cap-dependent and -independent protein synthesis occurred primarily at the translational level, as supported by DNA and mRNA transfection experiments. Concurrent with collapse of polyribosomes, viral mRNA was predominantly in 80s and lighter ribonucleoprotein fractions. We confirmed the accumulation of cytoplasmic granules in HeLa cells infected with the C7/K1 deletion mutant and further showed that viral mRNA was sequestered with SAMD9. RNA granules were still detected in G3BP KO U2OS cells, which remained non-permissive for the C7/K1 deletion mutant. Inhibition of cap-dependent and internal ribosome entry site mediated translation, sequestration of viral mRNA, and failure of PKR, RNase L or G3BP KO cells to restore protein synthesis support an unusual mechanism of host restriction. IMPORTANCE A dynamic relationship exists between viruses and their hosts in which each ostensibly attempts to exploit the other's vulnerabilities. A window is opened into the established condition, which evolved over millennia, if loss-of-function mutations occur in either the virus or host. Thus, the inability of viral host-range mutants to replicate in specific cells can be overcome by identifying and inactivating the opposing cellular gene. Here, we investigated a C7/K1 host range mutant of vaccinia virus in which the cellular gene SAMD9 serves as the principal host restriction factor. Host restriction was triggered early in infection and manifested as a block in translation of viral mRNAs. Features of the block include inhibition of cap-dependent and internal ribosome entry site-mediated translation, sequestration of viral RNA and inability to overcome the inhibition by inactivation of protein kinase R, ribonuclease L or G3 binding proteins, suggesting a novel mechanism of host restriction.