Two regions of the P protein are required to be active with the L protein for human parainfluenza virus type 1 RNA polymerase activity.
The paramyxovirus P protein is an essential component of the viral RNA polymerase composed of P and L proteins. In this study, we characterized the physical and functional interactions between P and L proteins using human parainfluenza virus type 1 (hPIV1) and its counterpart Sendai virus (SV). The hPIV1 P and SV L proteins or the SV P and hPIV1 L proteins formed complexes detected by anti-P antibodies. Functional analysis using the minigenome SV RNA containing CAT gene indicated that the hPIV1 P--SV L complex, but not the SV P--hPIV1 L complex, was biologically active. Mutant SV P or hPIV1 P cDNAs, which do not express C proteins, showed the same phenotype with wild-type P cDNAs, indicating that C proteins are not responsible for the dysfunction of SV P--hPIV1 L polymerase complex. Using the chimeric hPIV1/SV P cDNAs, we identified two regions (residues 387--423 and 511--568) on P protein, which are required for the functional interaction with hPIV1 L. These regions overlap with a previously identified domain for oligomer formation and binding to nucleocapsids. Our results indicate that in addition to a P--L binding domain, hPIV1 L requires a specific region on P protein to be biologically functional as a polymerase.