Two of the central issues in developing new strategies to interfere with viral infections concern the identification of cellular proteins involved in viral replication and/or antiviral measures and the dissection of the underlying molecular mechanisms. To gain initial insight into the role of host proteins in the life cycle of infectious bursal disease virus (IBDV), a double-stranded RNA virus, we examined the cellular nuclear factor 45 (NF45). NF45 was previously indicated to be involved in the replication process of other types of RNA viruses. Interestingly, by performing immunofluorescence studies, we found that in IBDV-infected cells the mainly nuclear NF45 accumulated at the sites of viral replication in the cytoplasm. NF45 was shown to specifically colocalize with the viral RNA-dependent RNA polymerase VP1, the capsid protein VP2, and the ribonucleoprotein VP3. Immunoprecipitation experiments indicated protein-protein associations between NF45 and VP1, VP2, and VP3. Expression of the individual VP3 or the combination of expression of VP1 and VP3 did not result in a cytoplasmic accumulation of NF45, which, among other data, showed that recruitment of the cellular protein in infected cells functionally correlates with the viral replication process. Since small interfering RNA(siRNA)-mediated downregulation of NF45 resulted in an approximately 5-fold increase of virus yield, our study suggests that NF45, by association with viral proteins, is part of a yet-uncharacterized cellular defense mechanism against IBDV infections.The highly contagious infectious bursal disease virus (IBDV), first described by Cosgrove (5), is the causative agent of the immunosuppressive infectious bursa disease (IBD) in young chickens. Worldwide, IBDV infections cause significant economic losses in the poultry industry. The high mortality rate of the host animals may be directly related to the virus infection. Moreover, IBDV originates lytic infections of proliferating B lymphocytes in the bursa of Fabricius that lead to a general humoral immunosuppression and reduced response to vaccines, which in turn favor respiratory and enteric diseases.IBDV is a nonenveloped, bisegmented (segment A and B) double-stranded RNA (dsRNA) virus belonging to the family Birnaviridae and the genus Avibirnavirus (7). Segment A contains two partially overlapping open reading frames (ORFs). The first ORF encodes the nonessential, nonstructural viral protein 5 (VP5) (33, 34, 51), which plays an important role in virulence and virus egress (28, 60). The second ORF encodes a polyprotein of 110 kDa that is autocatalytically cleaved by the viral protease VP4, yielding the pVP2 precursor (48 kDa) as well as VP4 (28 kDa) and VP3 (32 kDa) (3). Throughout virion maturation, pVP2 is further processed into the mature capsid protein VP2 (41 kDa) and four small peptides (6). While the largest peptide is autoproteolytically cleaved by VP2 (14), the other peptides are believed to be generated by trans-cleavage via the viral protease VP4 and/or cellular proteins (6). Segm...