Bioinformatic and Functional Characterization of Viral GADD34 Homologs
Cells respond to a variety of stresses by phosphorylating the alpha subunit of eukaryotic initiation factor 2 (eIF2α) and thereby shutting off cap-dependent protein synthesis. During viral infection, eIF2α is phosphorylated by the double-stranded RNA-activated protein kinase R (PKR) and the resulting translational shut off inhibits viral replication. To overcome the antiviral effects of PKR, viruses have evolved a repertoire of PKR inhibitors, many of which play a role in determining host tropism. In humans, eIF2α is dephosphorylated by two phosphatase complexes, containing paralogous protein phosphatase 1 (PP1) cofactors GADD34 and CReP, which target PP1 phosphatase activity by directly binding eIF2α. Herpes simplex virus (HSV) 1, African swine fever virus (ASFV) encode homologs of GADD34/CReP, which also form eIF2α dephosphorylation complexes with PP1. Here, we use basic local alignment (BLAST) searches to identify additional GADD34 homologs in 39 viruses from 10 different viral families. These viral GADD34 homologs are diverse in amino acid sequences, but all contain predicted PP1 and eIF2α binding motifs. Phylogenetic analysis shows that viral GADD34 homologs separate into two clades – a GADD34-like clade and a CReP-like clade. Additionally, several GADD34 homologs from arthropod infecting viruses clade with cellular GADD34 homologs from their host species, providing support for the idea that these viral GADD34 homologs are derived from host gene acquisition events. We also show that viral GADD34 homologs from HSV1, ASFV, canarypox virus, a newly isolated puffinpox virus, a fruitbat infecting herpesvirus, and an amphibian infecting picornavirus show some ability to counteract PKR in mammalian cells. More specifically, these GADD34 homologs rescue replication of a highly attenuated vaccinia virus (VACV) in PKR-competent cells. Lastly, there appears to be both virus and host cell line specificity in the anti-PKR activity of viral GADD34 homologs.