Functional analysis of the Cowpea mosaic virus movement protein

For successful infection, plant viruses must spread from an infected cell to neighboring uninfected cells. For this, Cowpea mosaic virus (CPMV) transports virus particles via tubules that are formed through plasmodesmata by the CPMV movement protein (MP). By using specific inhibitors, targeting of MP fused to GFP to the cell periphery was demonstrated not to depend on an intact cytoskeleton or functional secretion pathway. Studies with several mutant MPs in protoplasts and plants revealed that MP is first targeted to the plasma membrane and subsequently accumulates in highly dynamic peripheral punctate spots, and that tubules need to be disassembled in the neighboring cell to enable cell-to-cell movement of CPMV. Co-inoculation experiments with mutant and wild type MP indicated that separate domains of the MP are involved in targeting to the cell periphery and in tubule formation and that multimerization of MP subunits is necessary for targeting to punctate spots. Using affinity chromatography and a mutant MP it was found that MP is capable of specifically binding rGTP and that this binding plays a role in intracellular targeting of MP. Time-lapse microscopy, fluorescence resonance energy transfer and fluorescence recovery after photobleaching experiments have shown that tubules made by CPMV MP originate from peripheral punctate spots, consists of a highly organized MP-multimer and do not interact directly with the plasma membrane but probably via a host protein. Some of the results obtained in this study suggest a remarkable analogy between the assembly and disassembly of microtubules and tubules made by CPMV MP. Together with other results, this analogy was used to build a new model for CPMV cell-to-cell movement.