Charge changes near the N terminus of the coat protein of two potyviruses affect virus movement

Mutants of tobacco vein mottling virus (TVMV) with substitutions of Lys or Arg for Asp in the DAG motif at position 5 in the coat protein (CP) failed to infect tobacco plants systemically, but replicated and produced virions in protoplasts. Occasional systemic infections occurred when Nicotiana benthamiana or transgenic tobacco plants expressing wild-type TVMV CP were inoculated with these mutants, but viral progeny contained reversions to negatively or non-charged amino acids at position 5 or substitutions of Glu for Lys at position 8. The compensatory nature of these mutations was demonstrated by recreating one of the most common alterations. Tobacco etch virus (TEV) mutants with substitutions of Lys for Asp in the two DAG motifs near the CP N terminus also failed to infect tobacco plants systemically, and in situ histochemical analysis showed limited movement. A certain net charge evidently must be maintained near the CP N terminus for systemic movement to occur.