Identification and Characterization of the Helix-Destabilizing Activity of Rotavirus Nonstructural Protein NSP2

The rotavirus nonstructural protein NSP2 self-assembles into homomultimers, binds single-stranded RNA nonspecifically, possesses a Mg2+-dependent nucleoside triphosphatase (NTPase) activity, and is a component of replication intermediates. Because these properties are characteristics of known viral helicases, we examined the possibility that this was also an activity of NSP2 by using a strand displacement assay and purified bacterially expressed protein. The results revealed that, under saturating concentrations, NSP2 disrupted both DNA-RNA and RNA-RNA duplexes; hence, the protein possesses helix-destabilizing activity. However, unlike typical helicases, NSP2 required neither a divalent cation nor a nucleotide energy source for helix destabilization. Further characterization showed that NSP2 displayed no polarity in destabilizing a partial duplex. In addition, helix destabilization by NSP2 was found to proceed cooperatively and rapidly. The presence of Mg2+ and other divalent cations inhibited by approximately one-half the activity of NSP2, probably due to the increased stability of the duplex substrate brought on by the cations. In contrast, under conditions where NSP2 functions as an NTPase, its helix-destabilizing activity was less sensitive to the presence of Mg2+, suggesting that in the cellular environment the two activities associated with the protein, helix destabilization and NTPase, may function together. Although distinct from typical helicases, the helix-destabilizing activity of NSP2 is quite similar to that of the sigmaNS protein of reovirus and to the single-stranded DNA-binding proteins (SSBs) involved in double-stranded DNA replication. The presence of SSB-like nonstructural proteins in two members of the family Reoviridae suggests a common mechanism of unwinding viral mRNA prior to packaging and subsequent minus-strand RNA synthesis.