Tyrosine phosphorylation of the triple gene block protein 3 regulates cell-to-cell movement and protein interactions of Potato mop-top virus

Functions of viral proteins can be regulated through phosphorylation by serine/threonine kinases in plants, but little is known about the involvement of tyrosine kinases in plant virus infection. In this study, TGBp3, one of the three movement proteins encoded by a triple gene block (TGB) of Potato mop-top virus (PMTV), was detected for the first time in PMTV-infected plants and found to be tyrosine phosphorylated. Phosphorylation sites (Tyr87–89 and Tyr120) were located in two amino acid motifs con-served in the TGB-containing, rod-shaped plant viruses. Substitution of these tyrosine residues in both motifs was needed to abolish tyrosine phosphorylation of TGBp3. Substitution of Tyr87–89 with alanine residues enhanced the interaction between TGBp3 and TGBp2 and inhibited cell-to-cell movement of PMTV. On the other hand, substitution of Tyr120 with alanine re-sulted in no alteration in the interaction of TGBp3 with TGBp2, but the mutant virus was not infectious. The results suggest that tyrosine phosphorylation is a mechanism regulating the functions of plant virus movement proteins. Plant viruses encode movement proteins (MPs) to facilitate in-tra- and intercellular movement of viral genomes to and through plasmodesmata by recruiting the host trafficking systems (1–5).Much research has focused on themechanisms regulated by MPs, but less is known about regulation of MP activities. Protein phosphorylation causes a reversible posttranslational modifica-tion that plays a fundamental role in the regulation of many cel