Insights in luteovirid structural biology guided by chemical cross-linking and high resolution mass spectrometry.

Interactions among plant pathogenic viruses in the family Luteoviridae and their plant hosts and insect vectors are governed by the topology of the viral capsid, which is the sole vehicle for long distance movement of the viral genome. Previous application of a mass spectrometry-compatible cross-linker to preparations of the luteovirid Potato leafroll virus (PLRV; Luteoviridae: Polerovirus) revealed a detailed network of interactions between viral structural proteins and enabled generation of the first cross-linking guided coat protein models. In this study, we extended application of chemical cross-linking technology to the related Turnip yellows virus (TuYV; Luteoviridae: Polerovirus). Remarkably, all cross-links found between sites in the viral coat protein found for TuYV were also found in PLRV. Guided by these data, we present two models for the TuYV coat protein trimer, the basic structural unit of luteovirid virions. Additional cross-links found between the TuYV coat protein and a site in the viral protease domain suggest a possible role for the luteovirid protease in regulating the structural biology of these viruses.

The C terminus of the polerovirus p5 readthrough domain limits virus infection to the phloem.

Poleroviruses are restricted to vascular phloem tissues from which they are transmitted by their aphid vectors and are not transmissible mechanically. Phloem limitation has been attributed to the absence of virus proteins either facilitating movement or counteracting plant defense. The polerovirus capsid is composed of two forms of coat protein, the major P3 protein and the minor P3/P5 protein, a translational readthrough of P3. P3/P5 is required for insect transmission and acts in trans to facilitate long-distance virus movement in phloem tissue. Specific potato leafroll virus mutants lacking part or all of the P5 domain moved into and infected nonvascular mesophyll tissue when the source-sink relationship of the plant (Solanum sarrachoides) was altered by pruning, with the progeny virus now being transmissible mechanically. However, in a period of months, a phloem-specific distribution of the virus was reestablished in the absence of aphid transmission. Virus from the new phloem-limited infection showed compensatory mutations that would be expected to restore the production of full-length P3/P5 as well as the loss of mechanical transmissibility. The data support our hypothesis that phloem limitation in poleroviruses presumably does not result from a deficiency in the repertoire of virus genes but rather results from P3/P5 accumulation under selection in the infected plant, with the colateral effect of facilitating transmission by phloem-feeding aphid vectors.