Characterization of synthetic foot-and-mouth disease virus provirions separates acid-mediated disassembly from infectivity.

One of the final steps in the maturation of foot-and-mouth disease virus (FMDV) is cleavage of the VP0 protein to produce VP4 and VP2. The mechanism of this cleavage is unknown, but it is thought to function in stabilizing the virus particle and priming it for infecting cells. To investigate the cleavage process and to understand its role in virion maturation, we engineered synthetic FMDV RNAs with mutations at Ala-85 (A85) and Asp-86 (D86) of VP0, which border the cleavage site. BHK cells transfected with synthetic RNAs containing substitutions at position 85 (A85N or A85H) or at position 86 (D86N) yielded particles indistinguishable from wild-type (WT) virus in sedimentation and electrophoretic profiles. Viruses derived from these transfected cells were infectious and maintained their mutant sequences upon passage. However, BHK cells transfected with synthetic RNAs encoding Phe and Lys at these positions (A85F/D86K) or a Cys at position 86 (D86C) produced noninfectious provirions with uncleaved VP0 molecules. Despite their lack of infectivity, the A85F/D86K provirions displayed cell binding and acid sensitivity similar to those of WT virus. However, acid breakdown products of the A85F/D86K provirions differed in hydrophobicity from the comparable WT virion products, which lack VP4. Taken together, these studies are consistent with a role for soluble VP4 molecules in release of the viral genome from the endosomal compartment of susceptible cells.