Effects of mutations in the VP2/VP4 cleavage site of Swine vesicular disease virus on RNA encapsidation and viral infectivity

We studied VP0 cleavage of Swine vesicular disease virus (SVDV), a member of the Picornaviridae using a full-length cDNA copy of the Dutch SVDV isolate. The influences of mutations, introduced at the cleavage site of SVDV, on VP0 cleavage, RNA encapsidation and viral infection were studied. Double mutations at asparagine (VP0 aa 69) and serine (VP0 aa 70) resulted in no cleavage of VP0 and 100% inhibition of virus production. Mutation of the asparagine into threonine or phenylalanine resulted in a low amount of cleaved VP0 and infectious virus was found. After passage of this mutated virus VP0 cleavage became more efficient and the growth rate of the virus became similar to wild-type SVDV. The passaged virus had mutated at the asparagine site; the threonine had changed into an alanine and the phenylalanine into a cysteine. When the serine was mutated no maturation cleavage was observed and no infectious virus could be derived. All the mutations resulted in RNA encapsidation. We conclude that in the case of SVDV the cleavage site between VP2 and VP4 is essential for the formation of infectious virus which is comparable to poliovirus. The serine of the VP0 site was more important than the asparagine in this respect.