During cell–matrix adhesion, syndecan-4 transmembrane heparan sulphate proteoglycan plays a critical role in the formation of focal adhesions and stress fibres. We have shown previously that the syndecan-4 cytoplasmic domain directly binds to and activates PKC-α (protein kinase C-α) in vitro [Oh, Woods and Couchman (1997) J. Biol. Chem. 272, 8133–8136]. However, whether syndecan-4 has the same activity in vivo needs to be addressed. Using mammalian two-hybrid assays, we showed that syndecan-4 interacted with PKC-α in vivo and that this interaction was mediated through syndecan-4 cytoplasmic domain. Furthermore, the activation of PKC increased the extent of interaction between syndecan-4 and PKC-α. Overexpression of syndecan-4, but not a mutant lacking its cytoplasmic domain, specifically increased the level of endogenous PKC-α and enhanced the translocation of PKC-α into both detergent-insoluble and membrane fractions. In addition, rat embryo fibroblasts overexpressing syndecan-4 exhibited a slowed down-regulation of PKC-α in response either to a prolonged treatment with PMA or to maintaining cells in suspension culture. PKC-α immunocomplex kinase assays also showed that syndecan-4 overexpression increased the activity of membrane PKC-α. Taken together, these results suggest that syndecan-4 interacts with PKC-α in vivo and regulates its localization, activity and stability.

Abbreviations used: EGF, epidermal growth factor; ERK, extracellular-signal-regulated kinase; FBS, foetal bovine serum; FGF, fibroblast growth factor; bFGF, basic FGF; GFP, green fluorescent protein; HEK-293 cells, human embryonic kidney 293 cells; α-MEM, α-modified Eagle's medium; PDGF, platelet-derived growth factor; PIP2, phosphatidylinositol 4,5-bisphosphate; PKC, protein kinase C; RACK, receptor for activated PKC; REF, rat embryo fibroblast; TM4SF, transmembrane-4 superfamily.

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