Open reading frame (ORF) 2a of Sesbania mosaic virus (SeMV) codes for polyprotein 2a (Membrane anchor-protease-VPg-P10-P8). The C-terminal domain of SeMV polyprotein 2a was cloned, expressed and purified in order to functionally characterize it. The protein of size 8kDa (P8) domain, like viral protein genome linked (VPg), was found to be natively unfolded and could bind to nucleic acids. Interestingly, P10-P8 but not P8 showed a novel Mg(2+) dependent ATPase activity that was inhibited in the presence of poly A. In the absence of P8, the ATPase activity of the protein of size 10kDa (P10) domain was reduced suggesting that the natively unfolded P8 domain influenced the P10 ATPase.