Protein P4 of double-stranded RNA bacteriophage phi 6 is accessible on the nucleocapsid surface: epitope mapping and orientation of the protein.

Protein P4, an early protein of double-stranded RNA bacteriophage phi 6, is a component of the virion-associated RNA polymerase complex and possesses a nucleoside triphosphate (NTP) phosphohydrolase activity. We have produced and characterized a panel of 20 P4-specific monoclonal antibodies. Epitope mapping using truncated molecules of recombinant P4 revealed seven linear epitopes. The accessibility of the epitopes on the phi 6 nucleocapsid (NC) surface showed that at least the C terminus and an internal domain, containing the consensus sequence for NTP binding, protrude the NC shell. Four of the NC-binding antibodies distorted the integrity of the NC by releasing protein P4 and the major NC surface protein P8. This finding suggests a close contact between these two proteins. The dissociation of the NC led to the activation of the virion-associated RNA polymerase. The multimeric status of the recombinant P4 was similar to that of the virion-associated P4, indicating that no accessory virus proteins are needed for its multimerization.