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Effects of Movement Protein Mutations on the Formation of Tubules in Plant Protoplasts Expressing a Fusion Between the Green Fluorescent Protein and Cauliflower mosaic virus Movement Protein

Fusions between the green fluorescent protein (GFP) and the Cauliflower mosaic virus (CaMV) movement protein (MP) induce the formation of fluorescent foci and surface tubules in Arabidopsis thaliana leaf mesophyll protoplasts. Tubules elongate coordinately and progressively in an assembly process approximately 6 to 12 h following transfection of protoplasts with GFP-MP constructs. Tubules are not formed in protoplasts transfected by GFP-MP^ER2A, a MP mutation that renders CaMV noninfectious. A small number of short tubules are formed on protoplasts transfected by GFP-MP^N6 and GFP-MP^N13, two second-site revertants of ER2A that partially restore infectivity. Protoplasts cotransfected with cyan fluorescent protein (CFP)-MP^WT and GFP-MP^ER2A form tubules containing both MP fusions, indicating that although the GFP-MP^ER2A cannot induce tubule formation, GFP-MP^ER2A can coassemble or colocalize with CFP-MP^WT in tubules. Thus, CaMV MP-induced tubule formation in protoplasts correlates closely with the infectivity of mutation ER2A and its revertants, suggesting that tubule-forming capacity in plant protoplasts reflects a process required for virus infection or movement.