THE POTATO LEAFROLL VIRUS 17K MOVEMENT PROTEIN IS PBOSPHORYLATED BY AMEMBRANE-ASSOCIATED PROTEIN-KINASE FROM POTATO WITH BIOCHEMICAL FEATURES OF PROTEIN-KINASE-C

The 17 kDa protein (pr17), the phloem-limited movement protein (CLP) o f potato leafroll luteovirus (PLRV), is associated with membranous str uctures and localized to plasmodesmata [Tacke et al. (1993) Virology 1 97, 274-282; Schmitz, J. (1995) Ph.D. Thesis, University of Cologne]. In planta the protein is predominantly present in its phosphorylated f orm, 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 p reparations as wed as by commercially available kinases. A fusion prot ein containing the nucleic acid-binding, basic, C-proximal domain (pr1 7C1) was identified to be phosphorylated by a Ca2+- and phospholipid-d ependent, membrane-associated protein kinase. This protein kinase acti vity was inhibited by the addition of (19-36) protein kinase C (PKC) i nhibitory peptide, known to be a highly specific inhibitor of mammalia n PKC. Moreover, also the mammalian PKC from rat was able to phosphory late pr17 in vitro. The results suggest that phosphorylation of pr17 t akes place at membranous structures, possibly at the deltoid plasmodes mata connecting the sieve cell-companion cell complex of the phloem, b y the activity of PKC-related, membrane-associated protein kinase acti vity.