Syndecan-4 Proteoglycan Cytoplasmic Domain and Phosphatidylinositol 4,5-Bisphosphate Coordinately Regulate Protein Kinase C Activity*
- From the Department of Cell Biology, Cell Adhesion and Matrix Research Center and ‡Department of Neurobiology, University of Alabama, Birmingham, Alabama 35294
Abstract
Phosphatidylinositol 4,5-bisphosphate (PIP2) is involved in the organization of the actin cytoskeleton by regulating actin-associated proteins. The transmembrane heparan sulfate proteoglycan syndecan-4 also plays a critical role in protein kinase C (PKC) signaling in the formation of focal adhesions and actin stress fibers. The cytoplasmic domain of syndecan-4 core protein directly interacts with and potentiates PKCα activity, and it can directly interact with the phos- phoinositide PIP2. We, therefore, investigated whether the interaction of inositol phosphates and inositol phospholipids with syndecan-4 could regulate PKC activity. Data from in vitro kinase assays using purified PKCαβγ show that in the absence of phosphatidylserine and diolein, PIP2 increased the extent of autophosphorylation of PKCαβγ and partially activated it to phosphorylate both histone III-S and an epidermal growth factor receptor peptide. This activity was dose-dependent, and its calcium dependence varied with PKC isotype/source. Addition of the cytoplasmic syndecan-4 peptide, but not equivalent syndecan-1 or syndecan-2 peptides, potentiated the partial activation of PKCαβγ by PIP2, resulting in activity greater than that observed with phosphatidylserine, diolein, and calcium. This study indicates that syndecan-4 cytoplasmic domain may bind both PIP2 and PKCα, localize them to forming focal adhesions, and potentiate PKCα activity there.
Footnotes
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↵* This work was supported by National Institutes of Health Grants GM50194 (to J. R. C.) and MH50102 and HD32901 (to A. W. T.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵§ To whom correspondence should be addressed: Dept. of Cell Biology, University of Alabama, VH 201C, University Blvd., Birmingham, AL 35294-0019. Tel.: 205-934-2626; Fax: 205-975-9956
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↵1 The abbreviations used are: PIP2, phosphatidylinositol 4,5-bisphosphate; PIP3, phosphatidylinositol 3,4,5-triphosphate; PKC, protein kinase C; PS, phosphatidylserine; IP6, inositol hexaphosphate; IP4, inositol tetraphosphate; DL, diolein; EGF, epidermal growth factor; PL, phospholipid.
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↵2 J. R. Couchman, A. Woods, E.-S. Oh, G. Prestwich, and A. W. Theibert, unpublished observations.
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- Received December 19, 1997.
- The American Society for Biochemistry and Molecular Biology, Inc.
