Structural flexibility allows the functional diversity of potyvirus genome-linked protein VPg

Several viral genome-linked proteins (VPgs) of plant viruses are intrinsically disordered and undergo folding transitions in the presence of partners. This property has been postulated to be one of the factors that enable the functional diversity of the protein. We created a homology model of Potato virus A VPg and positioned the known functions and structural properties of potyviral VPgs on the novel structural model. The model suggests an elongated structure with a hydrophobic core composed of antiparallel β-sheets surrounded by helices and a positively charged contact surface where most of the known activities are localized. The model most probably represents the fold induced immediately after binding of VPg to a negatively charged lipid surface or to SDS. When the charge of the positive surface was lowered by lysine mutations, the efficiencies of in vitro NTP binding, uridylylation reaction, and unspecific RNA binding were reduced and in vivo the infectivity was debilitated. The most likely uridylylation site, Tyr63, locates to the positively charged surface. Surprisingly, a Tyr63Ala mutation did not prevent replication completely but blocked spreading of the virus. Based on the localization of Tyr119 in the model, it was hypothesized to serve as an alternative uridylylation site. Evidence to support the role of Tyr119 in replication was obtained which gives a positive example of the prediction power of the model. Taken together, our experimental data support the features presented in the model and the idea that the functional diversity is attributable to structural flexibility.