Joseph Curran

The template for paramyxovirus RNA synthesis is not naked RNA but the helical nucleocapsid core of the virus, in which each nucleocapsid protein (N protein) is predicted to be associated with precisely 6 nucleotides (nt) (11). Presumably as a consequence of this association, paramyxovirus genomes are replicated efficiently only when they are a multiple of 6 nt in length, and this has been dubbed the “rule of six” (4). The structure of paramyxovirus nucleocapsids is thus central to understanding how this rule might operate.

We present evidence that the formation of NP-P and P-L protein complexes is essential for replication of the genome of Sendai defective interfering (DI-H) virus in vitro, using extracts of cells expressing these viral proteins from plasmids. Optimal replication of DI-H nucleocapsid RNA required extracts of cells transfected with critical amounts and ratios of each of the plasmids and was three- to fivefold better than replication with a control extract prepared from a natural virus infection. Extracts in which NP and P proteins were coexpressed supported replication of the genome of purified DI-H virus which contained endogenous polymerase proteins, but extracts in which NP and P were expressed separately and then mixed were inactive. Similarly, the P and L proteins must be coexpressed for biological activity. The replication data thus suggest that two protein complexes, NP-P and P-L, are required for nucleocapsid RNA replication and that these complexes must form during or soon after synthesis of the proteins. Biochemical evidence in support of the formation of each complex includes coimmunoprecipitation of both proteins of each complex with an antibody specific for one component and cosedimentation of the subunits of each complex. We propose that the P-L complex serves as the RNA polymerase and NP-P is required for encapsidation of newly synthesized RNA.

Two domains involved in RNA synthesis have recently been found within the N-terminal 77 amino acids of the Sendai virus P protein. One domain is required for RNA synthesis per se and has properties in common with the transactivation domains of cellular transcription factors. The second domain is thought to be specifically required for the nascent chain assembly step in genome replication. We have further mapped this second domain by the construction of chimeric and deleted P proteins to amino acids 33 to 41 of P and by examining the abilities of these P proteins to support DI genome replication in vivo. Using glycerol gradient sedimentation, we have shown that this domain is required to form a stable complex with unassembled NP (P-NP0) and to prevent NP from assembling illegitimately, i.e., independently of the concurrent assembly of a nascent viral genome. Since the P-NP0 complex represents the functional form of unassembled NP which is delivered to the nascent chain during genome replication, and since amino acids 33 to 41 are not required for the stable interaction of P with the assembled NP of the nucleocapsid, this chaperone function of P is not required for mRNA synthesis or the RNA synthesis step of genome replication.