Assembly protein part of the packaging complex that packages the viral RNA segments, replicate them into a double-stranded form and transcribe them. Required for efficient procapsid assembly. Necessary for stable packaging. May stabilize the RNA-dependent RNA polymerase (RdRP) in its position at the three-fold axis on the inner side of empty-unexpanded procapsids. Could play a role in viral RNA recognition. Seems to be involved in the regulation of plus strand synthesis (transcription) as a fidelity factor. Homodimer. Part of the packaging complex composed of RDRP, P4 and P7. Interacts with RDRP (Probable). Found in the capsid (Probable). Prior to RNA packaging, localizes on the inner procapsid surface near the three-fold axis of symmetry.