Infectious Bursal Disease Virus Capsid Protein VP3 Interacts both with VP1, the RNA-Dependent RNA Polymerase, and with Viral Double-Stranded RNA

Abstract

Infectious bursal disease virus (IBDV) is a double-stranded RNA (dsRNA) virus of the Birnaviridae family. Its two genome segments are encapsidated together with multiple copies of the viral RNA-dependent RNA polymerase, VP1, in a single-shell capsid that is composed of VP2 and VP3. In this study we identified the domains responsible for the interaction between VP3 and VP1. Using the yeast two-hybrid system we found that VP1 binds to VP3 through an internal domain, while VP3 interacts with VP1 solely by its carboxy-terminal 10 amino acids. These results were confirmed by using a reverse-genetics system that allowed us to analyze the interaction of carboxy-terminally truncated VP3 molecules with VP1 in infected cells. Coimmunoprecipitations with VP1-and VP3-specific antibodies revealed that the interaction is extremely sensitive to truncation of VP3. The mere deletion of the C-terminal residue reduced coprecipitation almost completely and also fully abolished production of infectious virions. Surprisingly, these experiments additionally revealed that VP3 also binds to RNA. RNase treatments and reverse transcription-PCR analyses of the immunoprecipitates demonstrated that VP3 interacts with dsRNA of both viral genome segments. This interaction is not mediated by the carboxyterminal domain of VP3 since C-terminal truncations of 1, 5, or 10 residues did not prevent formation of the VP3-dsRNA complexes. VP3 seems to be the key organizer of birnavirus structure, as it maintains critical interactions with all components of the viral particle: itself, VP2, VP1, and the two genomic dsRNAs.Infectious bursal disease virus (IBDV) is the causative agent of a highly contagious disease of young chickens. IBDV multiplies rapidly in developing B lymphocytes in the bursa of Fabricius, leading to immunosuppression and increased susceptibility to other diseases. Of the two IBDV serotypes, only serotype 1 is pathogenic to chickens (23). IBDV belongs to the family Birnaviridae (17). Members of this family are characterized by a double-stranded RNA (dsRNA) genome consisting of two segments (A and B) that are packaged within a singleshell icosahedral capsid of 60 nm.The smaller genome segment, B, of IBDV (approximately 2.9 kb) encodes a single protein, viral protein 1 (VP1; 91 kDa). This protein is the putative RNA-dependent RNA polymerase (12). It has the intriguing feature of occurring in virions in two forms: a form in which it is covalently bound to the 5Ј ends of the genomic dsRNA segments (viral protein genome-linked [VPg]) (16) and a free-polypeptide form. The larger dsRNA segment, A (approximately 3.3 kb), contains two partly overlapping open reading frames (ORFs). The first ORF encodes nonstructural protein VP5 (17 kDa). This protein proved to be nonessential for viral replication and infection (33, 41). The second ORF encodes a 110-kDa polyprotein, which is autocatalytically cleaved to give pVP2 (48 kDa), VP4 (28 kDa), and VP3 (32 kDa). The viral protease VP4, through a catalytic site belonging to the Lon protease family, is r...

Genome Assembly and Particle Maturation of the Birnavirus Infectious Pancreatic Necrosis Virus

“…The external surface of the particle is formed of trimeric subunits of VP2 (23,39), and the innermost layer is formed by trimeric subunits of VP3, the viral dsRNA, VP1, and VP4 (4,18). VP3, as predicted earlier, interacts with both segments of genomic dsRNA through its carboxy-terminal region (33,64,65), which also binds VP1 (40,63,64). The association of VP3 with viral dsRNA was also observed after extensive low-salt treatment of IPNV virions (29).…”

“…In this scenario, as soon as it is synthesized, the newly replicated VPg-dsRNA may act as an initiation complex to trigger genome assembly by continuously nucleating capsid proteins. VP3 may play a key role in stabilizing the genomic dsRNA, where charged residues at its C terminus seem to be essential for this interaction, and to prompt proper particle assembly (9,47,64). In a proposed model for IBDV VLPs, VP3 needs to be activated by either genomic RNA or VP1 to FIG.…”

Infectious Bursal Disease Virus, a Non-enveloped Virus, Possesses a Capsid-associated Peptide That Deforms and Perforates Biological Membranes

“…Backbone atoms Heavy atoms (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15) 0.41 Ϯ 0.18 1.49 Ϯ 0.44 (17)(18)(19)(20)(21)(22) 0.36 Ϯ 0.17 0.92 Ϯ 0.30 (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22) 0.77 Ϯ 0.33 1.63 Ϯ 0.48 (18)(19)(20)(21)(22)(23)(24)(25)(26)(27) 0.87 Ϯ 0.39 1.14 Ϯ 0.40 (27)(28)(29)(30)…”

“…VP3, the second major viral protein, binds the RNAdependent RNA polymerase VP1 and the genomic dsRNA (19,20). Birnaviruses present characteristic peptides associated to the virus particle (21).…”

Intracellular Interference of Infectious Bursal Disease Virus

“…Previous information suggested that the inner capsid protein VP3 might be a suitable candidate in our search for an effective dominant-negative polypeptide. VP3 (28.8 kDa) controls the assembly of the major capsid polypeptide VP2 (7,8,10) and interacts with both the viral genome (4,14) and the virus-encoded RNA polymerase VP1 (5). Additionally, several VP3 functional domains have been mapped and characterized (4,5,7,14).…”