Crystal structure of Bombyx mori nucleopolyhedrovirus ORF75 reveals a pseudo-dimer of thiol oxidase domains with a putative substrate-binding pocket.

Bombyx mori nucleopolyhedrovirus (BmNPV) triggers the global shutdown of host silkworm gene expression and protein synthesis approximately 12-18 h post-infection. Genome sequence analysis suggests that BmNPV ORF75 could be a flavin adenine dinucleotide (FAD)-linked thiol oxidase essential for virion assembly and virus propagation. Here, we report the crystal structure of BmNPV ORF75 at 2.1 Å (0.21 nm). The structure of BmNPV ORF75 resembles that of the thiol oxidase domain of human quiescin thiol oxidase (QSOX), displaying a pseudo-dimer of canonical and non-canonical thiol oxidase domains. However, BmNPV ORF75 is further dimerized by its C-terminal canonical thiol oxidase domain. Within the unique quaternary structural arrangement, the FAD-binding pocket and the characteristic CXXC motif from each monomer is 35 Å (3.5 nm) away from that of its corresponding molecule, which suggests that BmNPV ORF75 might adopt a deviant mechanism from that of QSOX to catalyse disulfide bond formation. Our thiol oxidase activity assay on the point mutations of the conserved residues participating in FAD recognition reveals an aromatic cage next to the FAD isoalloxazine moiety for substrate binding. These data suggest that the thiol oxidase activity of BmNPV ORF75 could be critical to catalyse the formation of the disulfide bonds of certain BmNPV proteins essential for BmNPV virion assembly.