Phosphorylation of the Cytoplasmic Tail of Syndecan-4 Regulates Activation of Protein Kinase Cα

doi:10.1074/jbc.273.40.25548

Phosphorylation of the Cytoplasmic Tail of Syndecan-4 Regulates Activation of Protein Kinase Cα*

Arie Horowitz and
Michael Simons‡

From the Angiogenesis Research Center, Cardiovascular Division, Department of Medicine, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts 02215

Abstract

Syndecans are transmembrane proteoglycans capable of carrying both heparan and chondroitin sulfate chains. The cytoplasmic tail of syndecan-4 was recently reported to undergoin vivo phosphorylation on Ser¹⁸³ in the membrane-proximal part of the tail (Horowitz, A., and Simons, M. (1998)J. Biol. Chem. 273, 10914–10918). However, the functional consequences of this event remain unknown. The cytoplasmic tail of syndecan-4 is known to undergo multimerization and to activate protein kinase Cα (PKCα), with both events depending on the presence of the commonly occurring phospholipid phosphatidylinositol 4,5-bisphosphate (PIP₂). In the present investigation we found that phosphorylation of Ser¹⁸³ produced a 10-fold reduction in the ability of syndecan-4 to activate PKCα, without affecting its ability to bind the PKC. Because Ser¹⁸³ is adjacent to positively charged lysine groups that resemble PIP₂-binding regions in several other proteins, phosphorylation of this serine may affect the binding affinity of the syndecan-4 cytoplasmic tail to PIP₂. We found that the Ser¹⁸³-phosphorylated cytoplasmic tail of syndecan-4 has indeed a significantly lower affinity to PIP₂ compared with the nonphosphorylated tail. Furthermore, Ser¹⁸³phosphorylation abolished PIP₂-dependent oligomerization of syndecan-4 cytoplasmic tails. We conclude that Ser¹⁸³ phosphorylation regulates syndecan-4-dependent activation of PKCα by reducing the affinity to PIP₂ and inhibiting the oligomerization of syndecan-4 cytoplasmic tails. These results further support the role of syndecan-4 in signal transduction in endothelial cells.

Footnotes

↵* This work was supported in part by National Institutes of Health Grants R01 HL53793 and P50 HL56993 (to M. S.), National Institutes of Health Training Grant HL07374 (to A. H.), and American Heart Association Scientist Development Grant 9730282N (to A. H.).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.
↵‡ To whom correspondence should be addressed: Cardiovascular Div., Beth Israel Deaconess Medical Center, 330 Brookline Ave., Boston, MA 02215. Tel.: 617-667-5364; Fax: 617-975-5201; E-mail:msimons{at}bidmc.harvard.edu.
Abbreviations:

HS

heparan sulfate

bFGF

basic fibroblast growth factor

PAGE

polyacrylamide gel electrophoresis

PIP₂

phosphatidylinositol 4,5-bisphosphate

PKC

protein kinase C

PVDF

polyvinylidene fluoride

PS

phosphatidylserine

Tricine

N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine

RFPEC

Rat fat pad capillary endothelial cell(s).
- Received July 28, 1998.
- Revision received August 6, 1998.
The American Society for Biochemistry and Molecular Biology, Inc.