Grapevine fanleaf virus (GFLV) is one of a large class of plant viruses whose cell-to-cell transport involves the passage of virions through tubules composed of virus-encoded movement protein (MP). The tubules are embedded within modified plasmodesmata, but the mechanism of targeting of MP to these sites is unknown. To study intracellular GFLV MP trafficking, a green fluorescent protein-MP fusion (GFP:MP) was expressed in transgenic tobacco BY-2 suspension cells under the control of an inducible promoter. We show that GFP:MP is targeted preferentially to calreticulin-labeled foci within the youngest cross walls, where it assembles into tubules. During cell division, GFP:MP colocalizes in the cell plate with KNOLLE, a cytokinesis-specific syntaxin, and both proteins are linked physically, as shown by coimmunoprecipitation of the two proteins from the same microsomal fraction. In addition, treatment with various drugs has revealed that a functional secretory pathway, but not the cytoskeleton, is required for tubule formation. However, correct GFP:MP targeting to calreticulin-labeled foci seems to be cytoskeleton dependent. Finally, biochemical analyses have revealed that at least a fraction of the MP behaves as an intrinsic membrane protein. These findings support a model in which GFP:MP would be transported to specific sites via Golgi-derived vesicles along two different pathways: a microtubule-dependent pathway in normal cells and a microfilament-dependent default pathway when microtubules are depolymerized.
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