Nucleocapsid incorporation into parainfluenza virus is regulated by specific interaction with matrix protein

The paramyxovirus nucleoproteins (NPs) encapsidate the genomic RNA into nuc leocapsids, which are then incorporated into virus particles. We determined the protein-protein interaction between NP molecules and the molecular mec hanism required for incorporating nucleocapsids into virions in two closely related viruses, human parainfluenza virus type 1 (hPIV1) and Sendai virus (SV). Expression of NP from cDNA resulted in in vivo nucleocapsid formatio n. Electron micrographs showed no significant difference in the morphologic al appearance of viral nucleocapsids obtained from lysates of transfected c ells expressing SV or hPIVI NP cDNA. Coexpression of NP cDNAs from both vir uses resulted in the formation of nucleocapsid composed of a mixture of NP molecules; thus, the NPs of both viruses contained regions that allowed the formation of mixed nucleocapsid, Mixed nucleocapsids were also detected in cells infected with SV and transfected with hPIV1 NP cDNA. However, when N P of SV was donated by infected virus and hPIV1 NP was from transfected cDN A, nucleocapsids composed of NPs solely from SV or solely from hPIVI were a lso detected. Although almost equal amounts of NP of the two viruses were f ound in the cytoplasm of cells infected with SV and transfected with hPIV1 NP cDNA, 90% of the NPs in the nucleocapsids of the progeny SV virions were from SV. Thus, nucleocapsids containing heterologous hPIV1 NPs were exclud ed during the assembly of progeny SV virions. Coexpression of hPIV1 NP and hPIV1 matrix protein (M) in SV-infected cells increased the uptake of nucle ocapsids containing hPIV1 NP; thus, M appears to be responsible for the spe cific incorporation of the nucleocapsid into virions. Using SV-hPIV1 chimer a NP cDNAs, we found that the C-terminal domain of the NP protein (amino ac ids 420 to 466) is responsible for the interaction with M.