The geminivirus BL1 movement protein is associated with endoplasmic reticulum-derived tubules in developing phloem cells

Plant viruses encode movement proteins that are essential for systemic infection of their host but dispensable for replication and encapsidation. BL1, one of the two movement proteins encoded by the bipartite geminivirus squash leaf curl virus, was immunolocalized to unique similar to 40-nm tubules that extended up to and across the walls of procambial cells in systemically infected pumpkin leaves, These tubules were not found in procambial cells from pumpkin seedlings inoculated with BL1 mutants that are defective in movement. The tubules also specifically stained with antisera to binding protein (BiP), indicating that they were derived from the endoplasmic reticulum. Independent confirmation of this endoplasmic reticulum association was obtained by subcellular fractionation studies in which BL1 was localized to fractions that contained both endoplasmic reticulum membranes and BiP. Thus, squash leaf curl virus appears to recruit the endoplasmic reticulum as a conduit for cell-to-cell movement of the viral genome.