SAMD9 Is an Innate Antiviral Host Factor with Stress Response Properties That Can Be Antagonized by Poxviruses

Abstract

We show that SAMD9 is an innate host antiviral stress response element that participates in the formation of antiviral granules. Poxviruses, myxoma virus and vaccinia virus specifically, utilize a virus-encoded host range factor(s), such as a member of the C7L superfamily, to antagonize SAMD9 to prevent granule formation in a eukaryotic initiation factor 2␣ (eIF2␣)-independent manner. When SAMD9 is stimulated due to failure of the viral antagonism during infection, the resulting antiviral granules exhibit properties different from those of the canonical stress granules. Poxviruses are enveloped large double-stranded DNA (dsDNA) viruses that replicate exclusively in the cytoplasm. Numbers of immunoregulatory factors are encoded in the poxvirus genome to antagonize multiple host defense pathways at all levels. We previously showed that a targeted knockout of M062R, an essential host range factor of the C7L superfamily in the myxoma virus (MYXV) genome, led to a profound defect of viral replication in cells from all species tested, including human, rabbit, and primate cells (1). Viral M062 antagonizes cellular SAMD9 during viral infection through direct protein-protein interactions, and knocking down the expression of SAMD9 in human cells lifts the block of late gene expression from MYXV-M062R-null infection (1). To investigate the antiviral functions of SAMD9, we examined the cellular localization of this host protein during permissive wild-type and nonpermissive MYXV-M062R-null infection by immunofluorescence (IF) staining. We observed that, during MYXV-M062R-null infection, SAMD9 formed a granule structure in the cytoplasm. The accumulation of concentrated SAMD9 was detected as early as 7 h postinfection (p.i.). By 18 h p.i., 91.0% (Ϯ6.2% standard deviation [SD]) of infected cells showed SAMD9 granules which were then mostly concentrated near the viral factories (Fig. 1A). During permissive infection by wild-type MYXV that expressed M062 protein, SAMD9 was observed to become distributed throughout the cells, which was also distinct from the exclusive cytoplasmic localization of SAMD9 in uninfected cells. We further investigated the SAMD9 granules and found that they are not conventional stress granules (SGs), as they cannot be dispersed by cycloheximide treatment (not shown) in a manner similar to that seen with the antiviral granules previously described, which form after infection with E3L-knockout vaccinia virus (VACV) (2). On the other hand, several hallmark markers of SGs, such as rasGTPase-activating protein-binding protein 1 (G3BP1) (3) (Fig. 1), T-cell intracellular antigen 1-related protein (TIAR) (4), USP10 (5), and key translation initiation factors such as eukaryotic initiation factor 4G (eIF4G) (6) (Fig. 1B), were readily detected surrounding the viral factories colocalized with the SAMD9 granules in infection by MYXV-M062-null. However, in addition to the SAMD9 granules that overlap G3BP1 staining, SAMD9 staining can be detected outside the viral factories. The polyclonal SAMD9 antibody used in thi...

Newcastle disease virus induces stable formation of bona fide stress granules to facilitate viral replication through manipulating host protein translation

“…Some viruses trigger atypical granules that exhibit distinct features compared with canonical SGs (20, 36). Unlike genuine SGs, AVGs do not disassemble in the presence of CHX, which stabilizes polysomes and prevents protein translation (21, 35).…”

Structural basis for antagonizing a host restriction factor by C7 family of poxvirus host-range proteins

“…An IFN-stimulated antiviral factor was discovered recently through studies of the C7 family of poxvirus host-range factors (11,14). Human alpha sterile motif domain-containing 9 (SAMD9) protein was identified as a protein that binds directly to MYXV-M62, and knocking down SAMD9 in human cells rescued the replication of M62R-deletion MYXV (11).…”

“…− (14,15). The exact function of SAMD9 is poorly understood, but it has antiviral activities against Japanese encephalitis virus (16), and its expression is induced by type I IFNs (17).…”

Quantitative Proteomic Analysis of Cell Responses to Electroporation, a Classical Gene Delivery Approach

“…It is an innate host antiviral stress response element that participates in the formation of antiviral granules. When SAMD9 is stimulated due to failure of the viral antagonism during infection, the resulting antiviral granules exhibit properties different from those of the canonical stress granules . It concludes that SAMD9 is a stress response element that can respond to both viral stimuli and environmental stimuli in both eIF2‐dependent and ‐independent manners.…”