PHOSPHORYLATION OF TOBACCO MOSAIC-VIRUS CELL-TO-CELL MOVEMENT PROTEINBY A DEVELOPMENTALLY REGULATED PLANT-CELL WALL-ASSOCIATED PROTEIN-KINASE

In host plants, cell-to-cell spread of tobacco mosaic virus (TMV) pres umably occurs through intercellular connections, the plasmodesmata. TM V movement is mediated by a specific virus-encoded single-strand nucle ic acid-binding protein, P30. The mechanism by which P30 operates is l argely unknown. Here, we demonstrate that P30 expressed in transgenic plants is a phosphoprotein. We have developed an assay for in vitro ph osphorylation of purified P30 by plant cell wall fractions and have lo calized the phosphorylation sites to amino acid residues Ser-258, Thr- 261, and Ser-265. Interestingly, the P30 phosphorylation sites do not correspond to any known consensus phosphorylation sites for protein ki nases. While P30 binding to single-stranded DNA (ssDNA) was shown to i nvolve Thr-201, phosphorylation of this residue does not appear to pla y a role in binding activity. The protein kinase activity contained in the cell wall fractions was developmentally regulated, expressed pred ominantly in leaves. Within a leaf, this protein kinase activity incre ased with leaf maturation and correlated with the reported development of secondary plasmodesmata, sites of P30 accumulation. We suggest tha t phosphorylation may represent a mechanism for the host plant to sequ ester P30 following its localization to cell walls.