Determinants of duck Tembusu virus NS2A/2B polyprotein procession attenuated viral replication and proliferation in vitro.

Duck Tembusu virus (DTMUV), a mosquito-borne Flavivirus, has caused serious economic losses for the Chinese poultry industry. The genome is translated into a polyprotein that is cleaved to mature protein by host and viral proteases in the host cell, and this proteolytic process is important for the viral life cycle. However, the cleavage mechanism of DTMUV polyprotein is still unclear. In this study, we identified that several amino acids (P1-R, P1'-G, P2-R, P3-T, and P4-V) were vital for NS2A/2B cleavage. Meanwhile, both NS2A and NS2B were essential in cis for polyprotein NS2A/2B intramolecular cleavage. Subsequently, a DTMUV replicon and an infectious clone showed that the P1 site is essential to viral replication, while a mutation in P1' could boost viral RNA replication. Furthermore, a recombinant virus with P1 and P1' site mutations named rDTMUV-NS2A/2B-P1P1'(AA) was rescued from transfected BHK21 cells. The maximum viral titers and viral genome copies of rDTMUV-NS2A/2B-P1P1'(AA) were much lower than those of rDTMUV-WT both in the intracellular and extracellular samples of transfected and infected BHK21 cells. Taken together, the NS2A/2B cleavage sites processed by the NS2B3 protease are vital for DTMUV proliferation and virulence.