Peptides resulting from the pVP2 C-terminal processing are present in infectious pancreatic necrosis virus particles

This study investigates the proteolytic processing of infectious pancreatic necrosis virus (IPNV), a birnavirus that causes disease in farmed fish. The researchers used mass spectrometry and N-terminal sequencing to analyze purified IPNV particles and identified peptides derived from the C-terminal region of pVP2, the precursor to the major capsid protein VP2. They detected three primary peptides (residues 443–486, 487–495, and 496–508 of the viral polyprotein) and fourteen additional peptides generated through further processing of two of these fragments. The viral protease VP4 appears responsible for generating the primary peptides through specific cleavage sites, while a carboxypeptidase likely produces the additional smaller peptides. By expressing a truncated polyprotein with a stop codon at position 443, the researchers confirmed that mature VP2 extends from residues 1–442. These findings demonstrate that IPNV, like other birnaviruses (IBDV and BSNV), incorporates multiple virus-derived peptides into its virion particles as part of normal maturation, suggesting this is a fundamental characteristic of the Birnaviridae family.

Key findings

• IPNV particles contain three peptides from pVP2 C-terminal processing (residues 443–486, 487–495, and 496–508) plus fourteen additional peptides generated from further proteolysis
• The VP4 viral protease cleaves at sites matching the motif [S/T]XAQA to generate primary peptides, while a carboxypeptidase produces additional smaller fragments
• Mature VP2 consists of residues 1–442 of the polyprotein, with an acetylated N-terminus
• The presence of multiple structural peptides in virions is characteristic of birnaviruses, indicating this is a conserved feature of the family
• VP2 can self-assemble into particles approximately 25 nm in diameter when released from disrupted virions