Protein kinase Cdc7 supports viral replication by phosphorylating Avibirnavirus VP3 protein

Phosphorylation is an important post-translational modification that has crucial roles in diverse cellular biological pathways and in various viral life cycles. Here, we report that the VP3 protein, the major inner capsid protein of Avibirnavirus infectious bursal disease virus (IBDV), regulates viral replication by phosphorylation during infection. Our data showed that IBDV infection induced the upregulation of Cdc7 expression, followed by phosphorylation of residue serine 13 (S13) on VP3 by interaction. Virus rescue showed that phosphorylation of VP3 at site S13 enhanced the replication ability of IBDV. Moreover, further study demonstrated that the increase of IBDV replication was relevant to the addition of negative charge of phosphoserine on VP3. Meanwhile, the knockdown of Cdc7 severely blocked the phosphorylation level of VP3 S13 and impaired the replication of wild-type but not S13A mutant recombinant IBDV. Thus, we demonstrated that Cdc7 was a crucial enzyme that phosphorylated IBDV VP3 at site serine 13 for viral proliferation. The Avibirnavirus infectious bursal disease virus is still an important agent which largely threatens global poultry farming industry economics. VP3 is a multifunctional scaffold structural protein that is involved in virus morphogenesis and the regulation of diverse cellular signaling pathways. However, little is known about the roles of VP3 phosphorylation during the IBDV life cycle. In this study, we determined that IBDV infection induced the upregulation of Cdc7 expression and phosphorylated the VP3 Ser13 site to promote viral replication. Moreover, we confirmed that the negative charge addition of phosphoserine on VP3 at the S13 site was essential for IBDV proliferation. This study provides novel insight into the molecular mechanisms of VP3 phosphorylation-mediated regulation of IBDV replication.