Functional analyses and identification of two arginine residues essential to the ATP-utilizing activity of the triple gene block protein 1 of bamboo mosaic potexvirus

The TGBp1 of bamboo mosaic potexvirus (BaMV) is encoded by the first overla pping gene of the triple-gene-block (TGB), whose products are thought to pl ay roles in virus movement between plant cells. This protein forms cytoplas mic inclusions associated with virus particles in the BaMV-infected tissues . It has been proposed that the inclusion is one of the active forms of TGB p1. To prove this idea, we purified the TGBp1 inclusions from both the BaMV -infected Chenopodium quinoa and Escherichia coli cells overexpressing this protein to test some of their biochemical activities. We found that the TG Bp1 inclusions isolated from the infected plant leaves, but not from E. col i, possess the NTP-binding and NTPase activities. However, they lack the RN A-binding activity possessed by the soluble TGBp1. These results indicate t hat the TGBp1 proteins in the BaMV-infected tissues assume two different fu nctional forms. Mutational analyses and competition experiments show that t he two arginine residues, Arg-16 and Arg-21, essential to RNA binding, are also required for the ATP-utilizing activity of the soluble TGBp1. This ind icates that a same-structure motif is required for the two functions of the soluble TGBp1. The location of the two arginine residues outside the seven conserved motifs of the NTP-utilizing superfamily I RNA helicases, to whic h TGBp1 belongs, suggests that an extra-structure motif, besides the seven conserved ones, is required for the NTP-utilizing activity of the TGBp1 pro tein of BaMV. (C) 2000 Academic Press.