Phosphorylations of the Abutilon Mosaic Virus Movement Protein Affect Its Self-Interaction, Symptom Development, Viral DNA Accumulation, and Host Range

Abstract

The genome of bipartite geminiviruses in the genus Begomovirus comprises two circular DNAs: DNA-A and DNA-B. The DNA-B component encodes a nuclear shuttle protein (NSP) and a movement protein (MP), which cooperate for systemic spread of infectious nucleic acids within host plants and affect pathogenicity. MP mediates multiple functions during intra-and intercellular trafficking, such as binding of viral nucleoprotein complexes, targeting to and modification of plasmodesmata, and release of the cargo after cell-to-cell transfer. For Abutilon mosaic virus (AbMV), phosphorylation of MP expressed in bacteria, yeast, and Nicotiana benthamiana plants, respectively, has been demonstrated in previous studies. Three phosphorylation sites (T221, S223, and S250) were identified in its C-terminal oligomerization domain by mass spectrometry, suggesting a regulation of MP by posttranslational modification. To examine the influence of the three sites on the self-interaction in more detail, MP mutants were tested for their interaction in yeast by twohybrid assays, or by Förster resonance energy transfer (FRET) techniques in planta. Expression constructs with point mutations leading to simultaneous (triple) exchange of T221, S223, and S250 to either uncharged alanine (MP AAA), or phosphorylation chargemimicking aspartate residues (MP DDD) were compared. MP DDD interfered with MP-MP binding in contrast to MP AAA. The roles of the phosphorylation sites for the viral life cycle were studied further, using plant-infectious AbMV DNA-B variants with the same triple mutants each. When co-inoculated with wild-type DNA-A, both mutants infected N. benthamiana plants systemically, but were unable to do so for some other plant species of the families Solanaceae or Malvaceae. Systemically infected plants developed symptoms and viral DNA levels different from those of wild-type AbMV for most virus-plant combinations. The results indicate a regulation of diverse MP functions by posttranslational modifications and underscore their biological relevance for a complex host plant-geminivirus interaction.

Variability, Functions and Interactions of Plant Virus Movement Proteins: What Do We Know So Far?

“…Among geminiviruses ( Table 1 ), the DNA from the nuclei is shuttled to the cytoplasm via coat protein (CP; V1 ORF) in monopartite geminiviruses (e.g., Begomoviruses and Mastreviruses), while in bipartite, it is through the DNA B-encoded nuclear shuttle protein (NSP/BV1) [ 85 , 86 ]. Subsequent cytoplasmic localization to PD and further intercellular movement in monopartite geminiviruses is through an MP encoded by the V2 ORF or V3 ORF (e.g., beet curly top virus, a Curtovirus) or by the DNA-B encoded MP (BC1) in bipartite ones.…”

“…Subsequent cytoplasmic localization to PD and further intercellular movement in monopartite geminiviruses is through an MP encoded by the V2 ORF or V3 ORF (e.g., beet curly top virus, a Curtovirus) or by the DNA-B encoded MP (BC1) in bipartite ones. Functional variation occurs in MP among monopartite viruses, for e.g., MPs in monopartite Begomoviruses has DNA-binding property, while MPs encoded by monopartite Mastreviruses do not bind to DNA [ 85 , 86 ]. Hence, it needs the viral CP to form a complex with the DNA, and the CP-DNA complex is then carried across PD by the MP.…”