CAPSID PROTEIN DETERMINANTS INVOLVED IN CELL-TO-CELL AND LONG DISTANCE MOVEMENT OF TOBACCO ETCH POTYVIRUS

Abstract The tobacco etch potyvirus (TEV) capsid protein (CP) is necessary for cell-to-cell and long distance transport of the virus in plants. In this study, the transport phenotypes of TEV mutants containing CPs with a substitution of the highly conserved Ser122 (termed S122W) within the core domain, or with a deletion of sequences encoding 17 amino acid residues comprising most of the variable C-terminal domain (ΔC), were analyzed. The S122W and ΔC mutant genomes were amplified to levels comparable to parental virus in protoplasts. The S122W mutant was encapsidation-defective, although in transgenic plants expressing wild-type CP a small number of virions were observed after prolonged incubation. Cells infected by the ΔC mutant produced virions, indicating that the C-terminal domain is not necessary for encapsidation. The mutants exhibited unique defects in cell-to-cell and long distance movement in plants. The S122W mutant was confined to single, primarily inoculated epidermal cells in nontransgenic plants, but the cell-to-cell movement defect was rescued efficiently by transgenic CP. Long distance movement of this mutant was also rescued in transgenic plants, but accumulation in systemically infected tissue was low compared to parental virus. The ΔC mutant exhibited a slow cell-to-cell movement phenotype in inoculated leaves and a complete inability to move systemically in nontransgenic plants. Transgenic CP was able to rescue partially the slow cell-to-cell movement defect of the ΔC mutant, but not the long distance transport defect. Taken together with previous results, these data suggest that the core domain of TEV CP provides a function essential during cell-to-cell movement and that the variable N- and C-terminal regions exposed on the virion surface are necessary for long distance transport. In addition, trans -inhibition models are presented to account for the widely differing transgenic complementation efficiencies of the various movement-defective mutants.