To catalyze RNA synthesis, the Sendai virus P-L RNA polymerase complex first binds the viral nucleocapsid (NC) template through an interaction of the P subunit with NP assembled with the genome RNA. For replication, the polymerase utilizes an NP(0)-P complex as the substrate for the encapsidation of newly synthesized RNA which involves both NP-RNA and NP-NP interactions. Previous studies showed that the C-terminal 124 amino acids of NP (aa 401-524) contain the P-NC binding site. To further delineate the amino acids important for this interaction, C-terminal truncations and site-directed mutations in NP were characterized for their replication activity and protein-protein interactions. This C-terminal region was found in fact to be necessary for several different protein interactions. The C-terminal 492-524 aa were nonessential for the complete activity of the protein. Deletion of amino acids 472-491, however, abolished replication activity due to a specific defect in the formation of the NP(0)-P complex. Binding of the P protein of the polymerase complex to NC required aa 462-471 of NP, while self-assembly of NP into NC required aa 440-461. Site-directed mutations from aa 435 to 491 showed, however, that the charged amino acids in this region were not essential for these defects.