Measles virus (MeV), like all viruses of the order Mononegavirales, utilizes a complex consisting of genomic RNA, nucleoprotein, the RNA-dependent RNA polymerase, and a polymerase cofactor, the phosphoprotein (P), for transcription and replication. We previously showed that a recombinant MeV that does not express another viral protein, C, has severe transcription and replication deficiencies, including a steeper transcription gradient than the parental virus and generation of defective interfering RNA. This virus is attenuated in vitro and in vivo However, how the C protein operates and whether it is a component of the replication complex remained unclear. Here, we show that C associates with the ribonucleocapsid and forms a complex that can be purified by immunoprecipitation or ultracentrifugation. In the presence of detergent, the C protein is retained on purified ribonucleocapsids less efficiently than the P protein and the polymerase. The C protein is recruited to the ribonucleocapsid through its interaction with the P protein, as shown by immunofluorescence microscopy of cells expressing different combinations of viral proteins and by split luciferase complementation assays. Forty amino-terminal C protein residues are dispensable for the interaction with P, and the carboxyl-terminal half of P is sufficient for the interaction with C. Thus, the C protein, rather than being an "accessory" protein as qualified in textbooks so far, is a ribonucleocapsid-associated protein that interacts with P, thereby increasing replication accuracy and processivity of the polymerase complex.IMPORTANCE Replication of negative-strand RNA viruses relies on two components: a helical ribonucleocapsid and an RNA-dependent RNA polymerase composed of a catalytic subunit, the L protein, and a cofactor, the P protein. We show that the measles virus (MeV) C protein is an additional component of the replication complex. We provide evidence that the C protein is recruited to the ribonucleocapsid by the P protein and map the interacting segments of both C and P proteins. We conclude that the primary function of MeV C is to improve polymerase processivity and accuracy, rather than uniquely to antagonize the type I interferon response. Since most viruses of the Paramyxoviridae family express C proteins, their primary function may be conserved.

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