Context of the coat protein DAG motif affects potyvirus transmissibility by aphids

Previous work with tobacco vein mottling virus (TVMV) has established that a highly conserved three amino acid motif, asp-ala-gly (DAG), located near the N terminus of the coat protein (CP), is important for aphid transmission. However, several other potyviruses which have motifs other than DAG are aphid-transmissible. Creation of these motifs in TVMV through site-directed mutagenesis failed to render TVMV aphid-transmissible from infected plants, and the creation of a putative complementary motif in the helper component did not restore transmissibility. In an isolate of tobacco etch virus (TEV) that contains two consecutive DAG motifs separated by a single ala, transmissibility was abolished or reduced by mutations affecting the first motif, whereas mutations in the second motif had little or no effect. In a TEV mutant made non-transmissible due to an altered first motif, substitution of val for ala in the position immediately before the second DAG restored transmissibility, whereas changing val to ala in the location prior to the first DAG resulted in reduced TEV transmissibility. In contrast, a val to ala change in the position preceding the single DAG motif of TVMV did not affect transmission. Creation of another DAG motif at the beginning of the TVMV CP core, in a position where certain other potyviruses have a second DAG motif, did not restore transmissibility. Our results suggest that the mere presence of a DAG motif does not guarantee transmissibility and that the context in which the DAG or equivalent motif is found plays a role in the process.