A minimal region in the NTPase/helicase domain of the TGBp1 plant virus movement protein is responsible for ATPase activity and cooperative RNA binding

The TGBp1 protein, encoded in the genomes of a number of plant virus genera as the first gene of the 'triple gene block', possesses an NTPase/helicase domain characterized by seven conserved sequence motifs. It has been shown that the TGBp1 NTPase/helicase domain exhibits NTPase, RNA helicase and RNA-binding activities. In this paper, we have analysed a series of deletion and point mutants in the TGBp1 proteins encoded by Potato virus X (PVX, genus Potexvirus) and Poa semilatent virus (PSLV, genus Hordeivirus) to map functional regions responsible for their biochemical activities in vitro. It was found that, in both PVX and PSLV, the N-terminal part of the TGBp1 NTPase/helicase domain comprising conserved motifs I, Ia and II was sufficient for ATP hydrolysis, RNA binding and homologous protein-protein interactions. Point mutations in a single conserved basic amino acid residue upstream of motif I had little effect on the activities of C-terminally truncated mutants of both TGBp1 proteins. However, when introduced into the full-length NTPase/helicase domains, these mutations caused a substantial decrease in the ATPase activity of the protein, suggesting that the conserved basic amino acid residue upstream of motif I was required to maintain a reaction-competent conformation of the TGBp1 ATPase active site.