Disruption of microtubule organization and centrosome function by expression of Tobacco mosaic virus movement protein

The movement protein (MP) of Tobacco mosaic virus mediates the cell-to-cell transport of viral RNA throughplasmodesmata, cytoplasmic cell wall channels for direct cell-to-cell communication between adjacent cells.Previous in vivo studies demonstrated that the RNA transport function of the protein correlates with itsassociation with microtubules, although the exact role of microtubules in the movement process remainsunknown. Since the binding of MP to microtubules is conserved in transfected mammalian cells, we tookadvantage of available mammalian cell biology reagents and tools to further address the interaction inflat-growing and transparent COS-7 cells. We demonstrate that neither actin, nor endoplasmic reticulum (ER),nor dynein motor complexes are involved in the apparent alignment of MP with microtubules. Together withresults of in vitro coprecipitation experiments, these findings indicate that MP binds microtubules directly.Unlike microtubules associated with neuronal MAP2c, MP-associated microtubules are resistant to disruptionby microtubule-disrupting agents or cold, suggesting that MP is a specialized microtubule binding protein thatforms unusually stable complexes with microtubules. MP-associated microtubules accumulate ER membranes,which is consistent with a proposed role for MP in the recruitment of membranes in infected plant cells andmay suggest that microtubules are involved in this process. The ability of MP to interfere with centrosomal-tubulin is independent of microtubule association with MP, does not involve the removal of other testedcentrosomal markers, and correlates with inhibition of centrosomal microtubule nucleation activity. Theseobservations suggest that the function of MP in viral movement may involve interaction with the microtubulenucleatingmachinery.