A dysfunctional movement protein of Tobacco mosaic virus interferes with targeting of wild-type movement protein to microtubules

The Tobacco mosaic virus (TMV) movement protein (MP^TMV) mediates cell-to-cell viral trafficking by altering properties of the plasmodesmata (Pd) in infected cells. During the infection cycle, MP^TMV becomes transiently associated with endomembranes, microfilaments, and microtubules (MT). It has been shown that the cell-to-cell spread of TMV is reduced in plants expressing the dysfunctional MP mutant MP^NT-1. To expand our understanding of the MP function, we analyzed events occurring during the intracellular and intercellular targeting of MPTM and MPNx' when expressed as a fusion protein to green fluorescent protein (GFP), either by biolistic bombardment in a viral-free system or from a recombinant virus. The accumulation of MP^TMV:GFP, when expressed in a viral-free system, is similar to MP^TMV:GFP in TMV-infected tissues. Pd localization and cell-to-cell spread are late events, occurring only after accumulation of MP:GFP in aggregate bodies and on MT in the target cell. MP^NT-1:GFP localizes to MT but does not target to Pd nor does it move cell to cell. The spread of transiently expressed MP^TMV:GFP in leaves of transgenic plants that produce MP^NT-1 is reduced, and targeting of the MP^TMV:GFP to the cytoskeleton is inhibited. Although MP^TMV:GFP targets to the Pd in these plants, it is partially impaired for movement. It has been suggested that MP^NT-1 interferes with host-dependent processes that occur during the intracellular targeting program that makes MP movement competent.