Regulation of plasmodesmal transport by phosphorylation of tobacco mosaic virus cell-to-cell movement protein

Cell-to-cell spread of tobacco mosaic virus (TMV) through plant intercellul ar connections, the plasmodesmata, is mediated by a specialized viral movem ent protein (MP), In vivo studies using transgenic tobacco plants showed th at MP is phosphorylated at its C-terminus at amino acid residues Ser258, Th r261 and Ser265, When MP phosphorylation was mimicked by negatively charged amino acid substitutions, MP lost its ability to gale plasmodesmata, This effect on MP-plasmodesmata interactions was specific because other activiti es of MP, such as RNA binding and interaction with pectin methylesterases, were not affected, Furthermore, TMV encoding the MP mutant mimicking phosph orylation was unable to spread from cell to cell in inoculated tobacco plan ts. The regulatory effect of MP phosphorylation on plasmodesmal permeabilit y was host dependent, occurring in tobacco but not in a more promiscuous Ni cotiana benthamiana host. Thus, phosphorylation may represent a regulatory mechanism for controlling the TMV MP-plasmodesmata interactions in a host-d ependent fashion.