Phosphorylation of the TGBp1 movement protein of Potato virus X by a Nicotiana tabacum CK2-like activity

The movement protein (MP) TGBp1 of the potexvirus Potato virus X (PVX) is a multifunctional protein required for cell-to-cell movement within the host plant. Recent work on other plant viruses has indicated that MP phosphorylation by host kinases can regulate MP function. In this study, we demonstrate that recombinant and native TGBp1 are phosphorylated by Nicotiana tabacum extracts from both PVX-infected and non-infected leaves. The phosphorylation activity present in plant extracts has distinctive characteristics of casein kinase 2 (CK2): it is inhibited by heparin, stimulated by polylysine, and uses either ATP or GTP as phosphoryl donors. We also demonstrate that TGBp1 is efficiently phosphorylated by recombinant tobacco CK2 α subunit and by partially purified tobacco CK2. Phosphopeptide mass mapping reveals that TGBp1 is phosphorylated in Ser-165, which is localized within a CK2 consensus sequence. Our results strongly suggest that a N. tabacum kinase of the CK2 family is involved in TBGp1 phosphorylation during the course of viral infection.