An alanine residue in human parainfluenza virus type 3 phosphoprotein is critical for restricting excessive N 0 -P interaction and maintaining N solubility.

The phosphoprotein (P) of human parainfluenza virus type 3 (HPIV3) plays a pivotal role in viral RNA synthesis, which interacts with the nucleoprotein (N) to form a soluble N0 -P complex (N0 , free of RNAs) to prevent the nonspecific RNA binding and illegitimate aggregation of N. Functional regions within P have been studied intensively. However, the precise site (s) within P directly involved in N0 -P interaction still remains unclear. In this study, using a series of deleted and truncated mutants of P of HPIV3, we demonstrate that amino-terminal 40 amino acids (aa) of P restrict and regulate N0 -P interaction. Furthermore, using in vivo HPIV3 minigenome replicon assay, we identify a critical P mutant (PA28P ) located in amino-terminal 40 aa, which fails to support RNA synthesis of HPIV3 minigenome replicon. Although PA28P maintains an enhanced N-P interaction, it is unable to form N0 -P complex and keep N soluble, thus, resulting in aggregation and functional abolishment of N-P complex. Moreover, we found that recombinant HPIV3 with mutation of A28P in P failed to be rescued. Taken together, we identified a residue within the extreme amino-terminus of P, which plays a critical role in restricting the excessively N-P interaction and keeping a functional N0 -P complex formation.