A tyrosine residue in the small nuclear inclusion protein of tobacco vein mottling virus links the VPg to the viral RNA.

The identity of the amino acid residue that links the VPg of the potyvirus tobacco vein mottling virus (TVMV) to the viral RNA was determined. 32P-labeled TVMV RNA was digested with RNase A and micrococcal nuclease. The resulting 32P-labeled VPg was isolated and partially hydrolyzed with 6 N HCl at 110 degrees C for 2 h. Analysis by thin-layer electrophoresis revealed the presence of [32P]phosphotyrosine but not [32P]phosphoserine or [32P]phosphothreonine. Another preparation of TVMV RNA was treated with endoproteinase Lys-C, and the resulting peptide-RNA was purified by sodium dodecyl sulfate-sucrose gradient centrifugation. The sequence of the N-terminal 15 amino acid residues of the peptide, when compared with the RNA-derived amino acid sequence of the TVMV polyprotein, demonstrated that the peptide occurs in the small nuclear inclusion protein. These data suggest that Tyr-1860 of the polyprotein is the amino acid residue that links the TVMV VPg to the viral RNA.