Binding of monoclonal antibodies to the movement protein (MP) of Tobacco mosaic virus: influence of subcellular MP localization and phosphorylation

Monoclonal antibodies (mAbs) to recombinant movement protein (MPREC) of Tobacco mosaic virus (TMV) were used to reveal the dependence of MP epitope accessibility to mAbs on subcellular MP localization and post-translational MP phosphorylation. Leaves of Nicotiana benthamiana or N. tabacum were inoculated mechanically with TMV or agroinjected with an MP expression vector. At different time post-inoculation, ER membrane- and cell wall-enriched fractions (ER-MP and CW-MP, respectively) were isolated and analysed. The N-terminal region (residues 1-30) as well as regions 186-222 and 223-257 of MP from the CW and ER fractions were accessible for interaction with mAbs By contrast, the MP regions including residues 76-89 and 98-129 were not accessible. The C-terminal TMV MP region (residues 258-268) was inaccessible to mAbs not only in CW-MP, but also in ER-MP fractions Evidence is presented that phosphorylation of the majority of TMV MP C-terminal sites occurred on ER membranes at an early stage of virus infection, i.e not after, but before reaching the cell wall. C-terminal phosphorylation of purified MPREC abolished recognition of C-proximal residues 258-268 by specific mAbs, which could be restored by MP dephosphorylation Likewise, accessibility to mAbs of the C-terminal MP epitope in ER-MP and CW-MP leaf fractions was restored by dephosphorylation Substitution of three or four C-terminal Ser/Thr residues with non-phosphorylatable Ala also resulted in abolition of interaction of mAbs with MP.