The interferon antagonistic activities of the V proteins from two strains of Newcastle disease virus correlate with their known virulence properties

Abstract

Newcastle disease virus (NDV) is an avian paramyxovirus that exists as hundreds of strains with widely different virulence properties. The NDV V protein exhibits interferon (IFN) antagonistic activity, which contributes to the virulence of the virus. The IFN-antagonistic activities of the V proteins from the avirulent strain La Sota and the moderately virulent strain Beaudette C (BC) were compared in an assay for the rescue of a recombinant NDV expressing the green fluorescent protein (NDV-GFP). Consistent with the virulence properties of the two viruses, the BC V protein exhibits a 4-fold greater ability to rescue replication of NDV-GFP than the La Sota V protein. Four amino acid differences in the C-terminal region of V, as well as the N-terminal region, contribute to the difference in IFN-antagonistic activity between the two V proteins.

Fig 1

BC V exhibits a four-fold greater ability than La Sota V to rescue growth of NDV-GFP. (A) DF1 cells were mock-transfected, or transfected with empty pCAGGS vector, La Sota V or BC V. After 24 h, the cells were infected with NDV-GFP virus and examined for fluorescence 24 h later. (B) The growth of NDV-GFP virus was quantitated by counting the number of fluorescent cells from three different fields. The average values and standard deviations (error bars) are shown for one experiment out of a total of four. The absolute numbers of fluorescent cells vary from one experiment to another but the relative activities are consistent. (C) DF1 cells were transfected with wt or mutant V plasmids. After 24 h, lysates were prepared and Western blot was performed using V18 antiserum. V protein levels standardized to actin were determined by densitometry and are expressed relative to wt (set at 100%). These data represent one out of at least two experiments.

Fig. 2

Schematic representation of the amino acid differences in the BC and LaSota V proteins. The figure shows the residues at the four positions in the C-terminal region that differ between the two proteins.

Fig 3

Effect of wt and mutated V proteins on the growth of NDV-GFP virus. DF1 cells were mock-transfected (mock), or transfected with empty vector (vector), La Sota wt or mutated V plasmids (panel A) or BC wt or mutated V plasmids (panel B). After 24 h, the cells were infected with NDV-GFP at a moi of 0.001 and examined for fluorescence 24 h later. In panels C and D, the percent expression relative to wt (open bars) is shown for mutated LaSota (panel C) and BC V proteins (panel D), along with the percent of wt rescue of growth of NDV-GFP virus (filled bars), based on the data in panels A and B. Expression levels were determined as described in the legend to Fig. 1C. These data represent one experiment out of at least two.