Functions of Herpesvirus-Encoded Homologs of the Cellular Ribonucleotide Reductase Large Subunit

ABSTRACT

Deoxyribonucleotides (dNTPs) are important for the efficient growth of DNA viruses. Therefore, many DNA viruses have strategies for the upregulation of cellular dNTP levels. Both α- and γ-herpesviruses encode functional homologs of cellular dNTP anabolic enzymes, including the class I ribonucleotide reductase (RNR) large (R1) and small (R2) subunits, whereas β-herpesviruses modulate host cells to induce genes that increase dNTP levels. Interestingly, β-herpesviruses still express the nonfunctional RNR R1 subunit. However, it is not clear why β-herpesviruses still carry inactive R1 homologs. Recently, the R1 homologs of herpesviruses have been shown to inhibit innate immune signaling pathways. In particular, both functional and nonfunctional R1 homologs target receptor-interacting protein kinase 1 (RIP1) and inhibit RIP1-mediated signaling pathways to promote viral replication. Here, we summarize recent findings on the activity of herpesviral R1 homologs and discuss their roles in the regulation of innate immune signaling pathways.