Thus, both formation mechanism and potential functions of intracellular VP4-related tubules during IBDV infection largely remain unclear. the last C-terminal hydrophobic and amyloidogenic stretch 238YHLAMA243 with two aggregation-prone alanine residues was found to be essential for its intracellular self-assembly. The assembled VP4 fibrils show significantly low solubility, subsequently, the deposition of highly assembled VP4 structures ultimately deformed the host cytoskeleton and nucleus, which was potentially associated with IBDV lytic infection. Importantly, the assembly of VP4 significantly reduced the cytotoxicity of protease activity in host cells which potentially prevent the premature cell death and facilitate viral replication. This study provides novel insights into the formation mechanism and biological functions of the protease-related fibrils. The protein aggregation plays critically pathogenic roles in a wide range of increasingly prevalent diseases, including amyloid-beta peptide and hyperphosphorylation of Tau protein in Alzheimers disease, alpha-synuclein protein in Parkinsons disease, islet amyloid polypeptide in type-II diabetes, prion protein in transmissible spongiform encephalopathies and mutant P53 protein in some cancers or infectious bursal disease virus (IBDV), a representative member of family, is highly contagious pathogen which can damage the precursors of antibody-producing B lymphocytes and cause severe immunosuppression and mortality in young chickens9. IBDV-encoded VP4 has been identified as a serine protease which utilizes a unique serine/lysine catalytic dyad mechanism to process the viral polyprotein precursor (NH2-pVP2-VP4-VP3-COOH) and capsid precursor pVP2, and play critical roles in virion assembly and maturation10,11. Recently, we further found that VP4 is a phosphoprotein with phosphorylated Sabinene sites 26S, 99Y and 162T during IBDV infection, and the phosphorylation of 99Y and 162T partially contributes to the cleavage of intermediate precursor VP4-VP3 polyprotein12. VP4 can also interact with host glucocorticoid-induced Sabinene leucine zipper and suppress expression of type I interferon in IBDV-infected HEK293T cells to facilitate IBDV replication13. During IBDV assembly, viral ribonucleoprotein VP3 and capsid precursor pVP2, together with viral dsRNA and viral polymerase VP1, are effectively encapsidated into virus-like particles14,15,16. However, VP4 protein forms 24C26?nm type-II tubules in the cytoplasm and nucleus of IBDV-infected cells which are different from type-I virion-related tubules and fails to be eventually encapsidated into infectious IBDV virions17. Recent studies using VP4 protein expressed in showed that VP4 possess the capacity to self-assemble into the tubular structures18, and assays revealed that VP4 tubules had much lower endopeptidase and protease activity than monomeric and dimeric VP419. Our recent study showed that 26S, 99Y and 162T phosphorylation of VP4 protein was nonessential for VP4 assembly12. Thus, both formation mechanism and potential functions of intracellular VP4-related tubules during IBDV infection largely remain unclear. Here we systematically characterized the dynamic assembly of intracellular VP4 tubules. Importantly, VP4 were convinced to possess an amyloidogenic peptide at the C-terminus which is critical for VP4 assembly. The assembled VP4 tubules show significantly low solubility, and the accumulating deposition of VP4 tubules eventually destroys host cytoskeleton and nucleus during late stage of IBDV infection. VP4 assembly initiated at early stage of IBDV infection greatly reduces the cytotoxic effects of protease on host cells, which potentially prevents premature cell death and facilitates viral replication. Results Intracellular assembly of viral protease VP4 is independent of cell types VP4 was previously reported to form tubule-like structures within IBDV-infected chicken embryo fibroblast cells (CEFs)17, but its unclear if the intracellular formation of tubular VP4 is Sabinene specific for certain type of cells. We characterized VP4 expression in various IBDV-infected cells, including primary CEFs, DF-1 cells, Vero cells, HEK293T cells (can self-assemble into tubule-like structures18 which are very similar to tubules found in IBDV-infected cells17. However, it SSI2 is unclear if other viral proteins or intracellular factors are involved in the VP4 assembly during IBDV replication. Firstly, we examined the subcellular relationship of tubule-like VP4 structures with other IBDV-encoded viral proteins in infected DF-1 cells by dual-staining IFAs. Co-localization analysis showed that the needle-like VP4 structures did not overlap.