The plum pox virus (PPV) protein CI is an RNA helicase, whose function in the virus replication is still unknown. Recently, an RNA binding domain was mapped to a region of the CI protein that includes the arginine-rich motif VI typical of RNA helicases of the superfamily SF2. In the present study, a second region involved in RNA binding activity of the CI protein has been identified. Northwestern assays with a series of maltosebinding protein fusions that contain different CI fragments showed that the RNA binding domain is located between residues 75 and 143. This segment contains the two most amino-terminal conserved domains of RNA helicases: I, involved in NTP binding, and Ia, of unknown function. The results can be explained in the context of a close interdependence between the protein regions involved in the NTPase and RNA binding activities that is expected for an RNA helicase.
