An Efficient Trans Complementation System for In Vivo Replication of Defective Poliovirus Mutants

The picornavirus genome encodes a large single polyprotein that is processed by viral proteases to form an active replication complex. The replication complex is formed with the viral genome, host proteins, and viral proteins that are produced/translated directly from each of the viral genomes (cis-provided viral proteins). Efficient complementation in vivo of replication complex formation by trans-provided viral proteins, thus exogenous or ectopically expressed viral proteins, remains to be demonstrated. Here we report an efficient trans-complementation system for the replication of defective poliovirus (PV) mutants by a viral polyprotein precursor in HEK 293 cells. Viral 3AB in the polyprotein, but not 2BC, was processed exclusively in cis. Replication of a defective PV replicon mutant, with a disrupted cleavage site for viral 3Cpro protease between 3Cpro and 3Dpol (3C/D[A/G] mutant) could be rescued by a trans-provided viral polyprotein. Only a defect of 3Dpol activity, of the replicon could be rescued in trans; inactivating mutations in 2CATPase/hel, 3B, and 3Cpro of the replicon completely abrogated the trans-rescued replication. An intact N-terminus of the 3Cpro moiety, of the trans-provided 3CDpro, was essential for the trans-rescue activity. By using this trans-complementation system, a high-titer defective PV pseudovirus (> 107 infectious units per mL) could be produced with the defective mutants, whose replication was completely dependent on trans-complementation. This work reveals potential roles of exogenous viral proteins in PV replication and offers insights into protein/protein and protein/RNA interactions during picornavirus infection.