Expression and characterization of RNA-dependent RNA polymerase of Ectropis obliqua virus

The nonstructural protein 2C of a Picorna-like virus displays nucleic acid helix destabilizing activity that can be functionally separated from its ATPase activity

Picorna-like viruses in the Picornavirales order are a large group of positive-strand RNA viruses that include numerous important pathogens for plants, insects, and humans. In these viruses, nonstructural protein 2C is one of the most conserved proteins and contains ATPase activity and putative RNA helicase activity. Here we expressed 2C protein of Ectropis obliqua picorna-like virus (EoV; genus Iflavirus, family Iflaviridae, order Picornavirales) in a eukaryotic expression system and determined that EoV 2C displays ATP-independent nucleic acid helix destabilizing and strand annealing acceleration activity in a concentration-dependent manner, indicating that this picornaviral 2C is more like an RNA chaperone than like the previously predicted RNA helicase. Our further characterization of EoV 2C revealed that divalent metal ions, such as Mg(2+) and Zn(2+), inhibit 2C-mediated helix destabilization to different extents. Moreover, we determined that EoV 2C also contains ATPase activity like that of other picornaviral 2C proteins and further assessed the functional relevance between its RNA chaperone-like and ATPase activities using mutational analysis as well as their responses to Mg(2+). Our data show that, when one of the two 2C activities was dramatically inhibited or almost abolished, the other activity could remain intact, showing that the RNA chaperone-like and ATPase activities of EoV 2C can be functionally separated. This report reveals that a picorna-like virus 2C protein displays RNA helix destabilizing and strand annealing acceleration activity, which may be critical for picornaviral replication and pathogenesis, and should foster our understanding of picorna-like viruses and viral RNA chaperones.

Identification and characterization of Iflavirus 3C-like protease processing activities

Viral replication and capsid assembly in the viruses in the order Picornavirales requires polyprotein proteolytic processing by 3C or 3C-like (3CL) proteases. We identified and characterized the 3CL protease of Ectropis obliqua virus (EoV) of the newly established family Iflaviridae (order Picornavirales). The bacterially expressed EoV 3CL protease domain autocatalytically released itself from larger precursors by proteolytic cleavage, and cleavage sites were determined via N-terminal sequencing of the cleavage products. This protease also mediated trans-proteolytic activity and cleaved the polyprotein at the same specific positions. Moreover, we determined the critical catalytic residues (H2261, D2299, C2383) for the protease activity, and characterized the biochemical properties of EoV 3CL and its responses to various protease inhibitors. Our work is the first study to identify an iflaviral 3CL protease and further characterize it in detail and should foster our understanding of EoV and other iflaviruses.