Effect of truncation of the N-terminal region of the viral hemorrhagic septicemia virus (VHSV) P protein on viral replication

The phosphoprotein (P) of viral hemorrhagic septicemia virus (VHSV) plays an essential role in viral replication by interconnecting the L protein and the N protein-RNA complex. In this study, to investigate the role of the N-terminal region of the P protein in viral replication, we mutated the first or the first and second or the first, second, and third ATG codon into TGA stop codons. The respective mutants were named P1, P2, and P3. Recombinant VHSVs containing each mutated P gene (rVHSV-P1, -P2, and -P3) were successfully generated by supplying the intact P protein in trans. The rVHSV-P2 and -P3 were not generated from cells expressing truncated P proteins (P1, P2 or P3 protein), but the rVHSV-P1 produced infectious viruses, even in cells without any P-protein-expressing plasmids. Nucleotide sequence analysis of the P gene of rVHSV-P1 showed that a mutation had occurred that resulted in the fourth amino acid (isoleucine, ATT) being changed to methionine (ATG) without a frameshift (P0.5), suggesting that strong selection pressure might facilitate mutations that are advantageous or essential for virus replication. Infectious rVHSV-P2 and -P3 were produced in cells expressing the P0.5 protein, suggesting that the first three amino acids of the P protein of VHSV are dispensable for viral replication. Furthermore, although the P1 protein was shorter than the P0.5 protein by only two amino acid residues, no viruses were produced when the P1 protein was supplied indicating that the fourth and the fifth amino acid residues are indispensable for normal P protein functions involved in viral replication.