Phosphoprotein, P of human parainfluenza virus type 3 prevents self-association of RNA-dependent RNA polymerase, L

The RNA-dependent RNA-polymerase (RdRp) of human parainfluenza virus type 3 (HPIV3) is a large protein (L, 2233 amino acids), and along with the phosphoprotein (P, 603 amino acids) forms a heterocomplex that transcribes the genome RNA into mRNAs in vitro and in vivo that are 5′-capped and methylated and 3′-polyadenylated. The interaction of the P protein, an obligatory cofactor, imparts the RdRp activity of the L protein, which is otherwise inactive. The precise mechanism underlying this activation process remains unknown. Several recent reports suggested that the L proteins of paramyxoviruses, when expressed alone, self-associate to form an oligomeric structure. The presumptive oligomerization domain lies in the N-terminal part of the L protein (for HPIV3, 889 amino acids). Here, we demonstrate that a series of N-terminally deleted L proteins as well as several truncated proteins that span different regions of the L protein can also efficiently co-immunoprecipitate the full length L protein. In addition, by several biochemical parameters, the L–L interaction was shown to form aggregates rather than oligomers. In contrast, when the P protein was co-expressed with the L protein, the former bound to a domain spanning the N-terminal 1060 amino acids of the latter, which prevented L–L self-association, resulting in the formation of structurally competent and functionally active RdRp.