Cell-to-cell movement of potato virus X: The role of p12 and p8 encoded bythe second and third open reading frames of the triple gene block

Potato virus X (PVX) requires three proteins, p25, p12, and p8, encoded by the triple gene block plus the coat protein (CP) for cell-to-cell movement. When each of these proteins was co-expressed with a cytosolic green fluore scent protein (GFP) in the epidermal cells of Nicotiana benthamiana by the microprojectile bombardment-mediated gene delivery method, only p12 enhance d diffusion of coexpressed GFP, indicating an ability to alter plasmodesmal permeability. p25, p12, and CP, expressed transiently in the initially inf ected cells, transcomplemented the corresponding movement-defective mutants to spread through two or more cell boundaries. Thus, these proteins probab ly move from cell to cell with the genomic RNA. In contrast, p8 only functi oned intracellularly and was not absolutely required for cell-to-cell movem ent. Since overexpression of p12 overcame the p8 deficiency, p8 appears to facilitate the functioning of p12, presumably by mediating its intracellula r trafficking. Considering the likelihood that p12 and p8 are membrane prot eins, it is suggested that intercellular as well as intracellular movement of PVX involves a membrane-mediated process.