Coronavirus infectious bronchitis virus non-structural proteins 8 and 12 form stable complex independent of the non-translated regions of viral RNA and other viral proteins

- Nsp3, nsp5, nsp8 and nsp14 of IBV interact with nsp12 in cells infected with a recombinant IBV expressing HA-tagged nsp12.
- Interaction between IBV nsp8 and nsp12 is not dependent on viral genomic or sub-genomic RNA and other viral proteins.
- The putative nsp8-binding domain on IBV nsp12 is mapped to its N-terminal and C-terminal regions.
- IBV nsp2, nsp5, nsp8, nsp9 and nsp10 exhibit differential binding affinity to the 5′- or 3′-UTR of the IBV genomic RNA.

The cleavage products from coronavirus polyproteins, known as the non-structural proteins (nsps), are believed to make up the major components of the viral replication/transcription complex. In this study, several nsps encoded by avian gammacoronavirus infectious bronchitis virus (IBV) were screened for RNA-binding activity and interaction with its RNA-dependent RNA polymerase, nsp12. Nsp2, nsp5, nsp8, nsp9 and nsp10 were found to bind to untranslated regions (UTRs), while nsp8 was confirmed to interact with nsp12. Nsp8 has been reported to interact with nsp7 and functions as a primase synthesizing RNA primers for nsp12. Further characterization revealed that nsp8-nsp12 interaction is independent of the UTRs of viral RNA, and nsp8 interacts with both the N- and C-terminal regions of nsp12. These results have prompted a proposal of how the nsp7-nsp8 complex could possibly function in tandem with nsp12, forming a highly efficient complex that could synthesize both the RNA primer and viral RNA during coronavirus infection.