MUTATIONAL ANALYSIS OF THE COAT PROTEIN N-TERMINAL AMINO-ACIDS INVOLVED IN POTYVIRUS TRANSMISSION BY APHIDS

The nature of the amino acids in the N-terminal 'DAGX' motif of the co at protein of tobacco vein mottling virus (TVMV) that have a direct ef fect on aphid transmissibility of the virion were further defined by s ite-directed mutagenesis. In the first position of the DAGX motif, Asp or Asn are required for aphid transmissibility. In the second positio n, the nonpolar residue Ala, but not the nonpolar Gly or Val or the po lar Thr and Ser, is compatible with transmissibility. In the third pos ition, the small, neutral, nonpolar Gly appears to be critical; even s ubstitution of Ala, with a minimal side-chain, drastically reduces tra nsmissibility. Although the amino acid following the DAG sequence is n ot highly conserved among potyviruses, the presence of an acidic Glu o r Asp residue at this position in the TVMV coat protein drastically re duces or abolishes aphid transmissibility. An attempt was made to test the hypothesis that trypsin cleavage of the N terminus is involved in the aphid inoculation process by destroying a trypsin cleavage site d ownstream from the DAGX motif. While the predicted decrease in transmi ssion occurred from infected plants, there was no effect on the transm ission of purified virus. Of the 23 mutations in the DAGX region of TV MV reported here and previously, only two, substitutions of Lys and Ar g for Asp, had a detectable adverse effect other than on aphid transmi ssibility. These, and perhaps other, residues near the N terminus func tion in some phase of the TVMV life cycle, in addition to aphid transm ission.