The 17 kDa protein (pr17), the phloem-limited movement protein (MP) of potato leafroll luteovirus (PLRV), is associated with membranous structures and localized to plasmodesmata [Tacke et al. (1993) Virology 197, 274-282; Schmitz, J. (1995) Ph.D. Thesis, University of Cologne]. In planta the protein is predominantly present in its phosphorylated form, but it is rapidly dephosphorylated during isolation under native conditions. In an effort to examine the nature of the protein kinase(s) involved in the phosphorylation reaction, pr17 deletion mutants were expressed as fusion proteins in a bacterial expression vector system and tested for their ability to be phosphorylated by potato membrane preparations as well as by commercially available kinases. A fusion protein containing the nucleic acid-binding, basic, C-proximal domain (pr17C1) was identified to be phosphorylated by a Ca2+- and phospholipid-dependent, membrane-associated protein kinase. This protein kinase activity was inhibited by the addition of (19-36) protein kinase C (PKC) inhibitory peptide, known to be a highly specific inhibitor of mammalian PKC. Moreover, also the mammalian PKC from rat was able to phosphorylate pr17 in vitro. The results suggest that phosphorylation of pr17 takes place at membranous structures, possibly at the deltoid plasmodesmata connecting the sieve cell-companion cell complex of the phloem, by the activity of PKC-related, membrane-associated protein kinase activity.