Three C-terminal phosphorylation sites in the Abutilon mosaic virus movement protein affect symptom development and viral DNA accumulation

The Abutilon mosaic virus (AbMV, Geminiviridae) DNA B component encodes a movement protein (MP), which facilitates viral transport within plants and affects pathogenicity. The presence of phosphorylated serine and threonine residues was confirmed for MP expressed in yeast and Nicotiana benthamiana by comparative Western blot analysis using phospho-amino acid- and MP-specific immunodetection. Mass spectrometry of yeast-derived MP identified three phosphorylation sites located in the C-terminal domain (Thr-221, Ser-223 and Ser-250). To assess their functional relevance in plants, several point mutations were generated in the MP gene of DNA B, which replace Thr-221, Ser-223 and Ser-250, either singly or in combinations, with either an uncharged alanine or a phosphorylation-mimicking aspartate residue. When co-inoculated with DNA A, all mutants were infectious. In systemically infected plants the symptoms and/or viral DNA accumulation were significantly altered for several of the mutants.