The syndecan transmembrane proteoglycans are involved in the organization of the actin cytoskeleton and have important roles as cell surface receptors during cell-matrix interactions. We have shown that the syndecan-4 cytoplasmic domain (4L) forms oligomeric complexes that bind to and stimulate PKCα activity in the presence of PtdIns(4,5)P2, emphasizing the importance of multimerization in the regulation of PKCα activation. Oligomerization of the cytoplasmic domain of syndecan-4 is regulated either positively by PtdIns(4,5)P2 or negatively by phosphorylation of serine 183. Phosphorylation results in reduced PKCα activity by inhibiting PtdIns(4,5)P2-dependent oligomerization of the syndecan-4 cytoplasmic domain. Data from NMR and gel-filtration chromatography show that the phosphorylated cytoplasmic domain (p-4L) exists as a dimer, similar to 4L, but not as higher-order oligomers. NMR analysis showed that the overall conformation of p-4L is a compact intertwined dimer with an unusually symmetric clamp shape, and its molecular surface is mostly positively charged. The two parallel strands form a cavity in the center of the dimeric twist. An especially marked effect of phosphorylation of the syndecan-4 cytoplasmic domain is a dramatic conformational change near the C2 region that ablates an interaction site with the PDZ domain of syntenin. Wound healing studies further suggest that syndecan-4 phosphorylation might influence cell migration behavior. We conclude that the phosphorylation (Ser183) of syndecan-4 can play a critical role as a molecular switch to regulate its functions through conformational change.
Bibliographical noteFunding Information:
This work was supported by the NRL program of MOST NRDP (M1-0203-00-0020) (to W.L.) Korean Research Foundation grants (KRF-99-042-D00096, KRF-2002-CP0327) and the Korea Science and Engineering Foundation (KOSEF) through the Protein Network Research Center at Yonsei University (to W.L.). This work is supported, in part, by the Brain Korea 21 Project. J.R.C. and J.R.W. are supported by Wellcome Trust Programme Grant 065940. P.Z. and E.M. were supported by The Flanders Interuniversity Institute for Biotechnology, the Concerted Actions Program (GOA/2001/06) and by a grant from the Fund for Scientific Research, Flanders. P.Z. is a Postdoctoral Fellow of the Fund for Scientific Research, Flanders, Belgium (F.W.O.).
- Phosphatidylinositol 4, 5-bisphosphate
- Solution structure