The N-terminal cysteine protease domain of rice stripe tenuivirus Pc1 possesses deubiquitinating enzyme activity.

Virus encoded deubiquitinating enzyme (DUB) plays important roles in viral replication and the regulation of host innate immunity. Bioinformatics-based analysis revealed the presence of an ovarian tumor (OTU) protease domain in the N terminus of rice stripe tenuivirus (RSV) Pc1. Many viral OTU domains have been reported to possess DUB activity, which suggests that RSV OTU probably also have DUB activity. To confirm this prediction, we first expressed and purified RSV OTU domain (the N-terminal 200 amino acids of Pc1) and its three mutants (D42A, C45A and H148A) from Escherichia coli and analyzed its DUB activity in vitro. The purified RSV OTU hydrolyzed both K48-linked and K63-linked polyubiquitin chains, indicating RSV OTU domain has DUB enzyme activity in vitro. The mutations of the predicted catalytic sites (Asp42, Cys45 and His148) resulted in the loss of DUB activity, demonstrating these three residues were required for enzyme activity. Then, RSV OTU and its mutants were expressed in insect cells and assayed their DUB activities in vivo by co-transfection with HA-tagged ubiquitin. RSV OTU dramatically reduced ubiquitin-conjugated cellular proteins compared to control and the mutants, showing that RSV OTU also displays DUB activity in vivo. Characterization of RSV OTU DUB enzyme activity and its key catalytic residues will facilitate the development of novel antiviral reagents against RSV.

Characterization of an endonuclease in rice stripe tenuivirus Pc1 in vitro.

Rice stripe tenuivirus (RSV) initiates its mRNA transcription by using the cap-snatching mechanism during which an endonuclease activity is required for the cleavage of host mRNA. In this study, we aim to characterize the endonuclease in RSV. Sequence alignment revealed the presence of a cap-snatching endonuclease domain in RSV Pc1. Expression and in vitro enzymatic activity assay demonstrated that this domain indeed had a manganese-dependent endonuclease activity. The enzyme could efficiently degrade ssRNA with preference for unstructured ssRNA, but not DNA. Mutations in the endonuclease domain allowed the identification of four key residues (D547, D567, E585 and K604). The endonuclease of RSV was similar but not identical to other known viral endonucleases, suggesting that RSV endonuclease may have some distinct catalytic characteristics.

Size-independent and noncooperative recognition of dsRNA by the Rice stripe virus RNA silencing suppressor NS3.

Plant and animal viruses employ diverse suppressor proteins to thwart the host antiviral reaction of RNA silencing. Many suppressors bind dsRNA with different size specificity. Here, we examine the dsRNA recognition mechanism of the Rice stripe virus NS3 suppressor using quantitative biochemical approaches, as well as mutagenesis and suppression activity analyses in plants. We show that dimeric NS3 is a size-independent, rather than small interfering RNA-specific, dsRNA-binding protein that recognizes a minimum of 9 bp and can bind to long dsRNA with two or more copies. Global analysis using a combinatorial approach reveals that NS3 dimer has an occluded site size of ∼ 13 bp on dsRNA, an intrinsic binding constant of 1 × 10(8) M(-1), and virtually no binding cooperativity. This lack of cooperativity suggests that NS3 is not geared to target long dsRNA. The larger site size of NS3, compared with its interacting size, indicates that the NS3 structure has a border region that has no direct contact with dsRNA but occludes a ∼ 4-bp region from binding. We also develop a method to correct the border effect of ligand by extending the lattice length. In addition, we find that NS3 recognizes the helical structure and 2'-hydroxyl group of dsRNA with moderate specificity. Analysis of dsRNA-binding mutants suggests that silencing of the suppression activity of NS3 is mechanistically related to its dsRNA binding ability.