Structural basis of syndecan-4 phosphorylation as a molecular switch to regulate signaling

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 PKC alpha activity in the presence of Ptdins(4,5)P-2, emphasizing the importance of multimerization in the regulation of PKCOC activation. Oligomerization of the cytoplasmic domain of syndecan-4 is regulated either positively by PtdIns(4,5)P-2 or negatively by phosphorylation of serine 183. Phosphorylation results in reduced PKC alpha activity by inhibiting Ptdlns(4,5)P-2-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. (c) 2005 Elsevier Ltd. All rights reserved.