Roles of the Picornaviral 3C Proteinase in the Viral Life Cycle and Host Cells

Sun, Di; Chen, Shun; Cheng, Anchun; Wang, Mingshu

Members of the picornavirus family synthesize positive-and negative-stranded RNA by relying on viral RNA-dependent RNA polymerase. The 3C pro alone and its precursors, namely 3ABC, 3BC, 3BCD and P3 (3ABCD), have been demonstrated to stimulate RNA synthesis at different levels in in vitro systems [30] [31] [32] [33] [34] . There are three RNA structures in the genome, which are important for RNA replication and which interact with 3C pro and its precursor ( Figure 3 ) [35] . These interactions are important for RNA replication in PV. First, a cloverleaf-like structure in the 5′-NCR of the viral RNA (5′CL) is important for RNA replication [36, 37] and binding between 3CD and stem-loop d as well as binding between poly(rc)-binding protein 2 (PCBP2) and stem-loop b are required for viral replication in PV [38] . Mutation (K12N/R13N) in the 3C-binding region of PV results in complete inhibition of (−)strand synthesis. Complementation assays provide evidence that P3 is the preferred precursor that interacts with 5′CL. Upon binding to RNA, P3 can recruit another P3 to release 3D and VPg for efficient replication [30] . Second, the cis-acting replication RNA element (cre) is also important for RNA replication. During initiation of RNA synthesis, cre interacts with two molecules of 3CD, as shown by the "slide back" model for VPgpUpU synthesis in PV, and 3CD stimulates the cretemplated VPg uridylylation reaction. Although less effective, the function of 3CD can also be performed by 3C pro , the 3BC precursor and the 3BCD precursor [31] [32] [33] . In FMDV, uridylylation of three VPgs can be enhanced by 3CD and in contrast to PV, the 3B33C and 3B1233C precursors of FMDV can serve as uridylylation substrates, even without added 3C pro or 3CD [39, 40] . Third, the 3′NTR-poly (A) tail is important for RNA replication. Interaction among 3CD, poly (A) binding-protein (PABP) and the poly (A) tail is required for negative-strand RNA synthesis to facilitate circularization of the RNA genome [41] . There are several viral and cellular proteins that have been found to bind to the 3′NCR of picornavirus genomic RNA. For PV, the 3AB and 3CD precursors have been reported to interact with the 3′NTR of genomic RNA [42] ; for HAV, the 3AB and 3ABC precursors have been demonstrated to bind to the 3′NCR of genomic RNA [34] .