V. Citovsky et al., PHOSPHORYLATION OF TOBACCO MOSAIC-VIRUS CELL-TO-CELL MOVEMENT PROTEINBY A DEVELOPMENTALLY REGULATED PLANT-CELL WALL-ASSOCIATED PROTEIN-KINASE, Genes & development, 7(5), 1993, pp. 904-910
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.