PROTEIN-KINASE-C PHOSPHORYLATES 2 OF THE 4 KNOWN SYNDECAN CYTOPLASMICDOMAINS IN-VITRO

The transmembrane heparan sulfate proteoglycans of the syndecan family are implicated to participate in several cellular reactions which are dependent on protein kinase C. We have used an in vitro assay to asse ss whether any of the known syndecans may become phosphorylated direct ly by protein kinase C. Peptides corresponding to the complete cytopla smic domains of rat syndecans 1 through 4 were used as substrates for the enzyme. The syndecan-2 (fibroglycan) and. syndecan-3 (N-syndecan) peptides were both found to be phosphorylated by protein kinase C with Kms of 15+/-3 mu M and 85+/-25 mu M, respectively, while the syndecan -1 and -4 peptides were not phosphorylated under the conditions used. The sites of in vitro phosphorylation for syndecans-2 and -3 were loca lized to ser-197 and ser-339, respectively. Thus, among 13 available s ites (serines and threonines) in the four peptides, two were selective ly modified by the enzyme. The specificity and the kinetics of the rea ctions indicate that the cytoplasmic domains of syndecan-2 and -3 are likely to be physiological substrates for protein kinase C.