The results presented in this paper demonstrate for the first time that NSs of GBNV-To (K) is a bifunctional enzyme. Bioinformatic analysis revealed the presence of Walker motif A and B. However, the Walker motif B precedes the motif A in this protein (Fig. 1) . It was of interest to examine if rNSs could bind to NTPs and cleave them. As shown in Fig. 3 , rNSs exhibited the Poly(A) stimulated ATPase activity. The activity was tenfold higher in the presence of poly(A) compared to that without poly(A). None of the NSs proteins from bunyaviridae family have been shown to possess this enzymatic activity. The product was not formed when immunodepleted rNSs was used, which ruled out the possibility of any contaminating bacterial protein (ATPase) co-purifying with rNSs. Further, rNSs could hydrolyze other NTPs and dATP ( Fig. 5 and table 1) and the ATPase activity of rNSs was inhibited by an ATP analog {adenosine 59-(b, c imido) triphosphate} (Fig. 4C) . These results suggested that NTPase/dATPase activity is an intrinsic property of rNSs. On the basis of structural data, it has been suggested that the binding of nucleic acid induces conformational changes in the protein, which, in turn, stabilizes the bound ATP molecules in a conformation that is required for rapid hydrolysis [22, 23] . It is therefore reasonable to suggest that the observed stimulatory effect of poly(A) on GBNV NSs ATPase activity may reflect similar conformational changes. Point mutation in Walker motif B (D159A) reduced the NTPase activity (Fig. 5) . NTPase activity is metal ion dependent. The D159, like in other NTP binding proteins, is probably responsible for co-ordination of the metal ion and therefore mutation of this residue affects the NTPase activity. However, the mutation of K189 to A abolished the NTPase activity completely. The conserved lysine residue in the Walker motif A is suggested to be important for interaction with b, c phosphate of NTP [21] . Therefore, mutation of this residue renders the protein inactive. The lysine residue in the GKT motif is shown to be critical for ATP hydrolysis in many other ATPases as well. For example, the mutation of lysine to glutamine in eIF-4A protein resulted in severely reduced ATP binding, and complete loss of ATPase activity [24] . Gross and Shuman [25] reported that an alanine substitution at the lysine residue of the GKT motif of NPH II decreased its NTPase activity by 1/20 of that of the wild type protein in the presence of RNA or DNA cofactors. The difference in the mutational effects at the same site in eIF-4A and NPHII could be a result of the nature of each protein and sensitivity of the specific experiments. Similar mutation has been performed with NS3 protein of Dengue virus type 2 [26] and HCV, which abolished the ATPase activity [27] .