earticle

영어

Syndecan-4, a transmembrane heparan sulfate proteoglycan, plays a critical role in cell adhesion as adhesion receptor. Both transmembrane and cytoplasmic domain of syndecan-4 are known to be involved in the regulation of syndecan-4 functions. In this study, we further investigated the domain hierarchy of syndecan-4. Fluorescence resonance energy transfer (FRET)-based assays showed that expression of syndecan-4 enhanced RhoA activation. RhoA activation was much lower in rat embryonic fibroblasts (REFs) plated on fibronectin-fragment lacking the heparin binding domain, than that of cells on fibronectin. Syndecan-4 transmembrane domain mutants that were defective in transmembrane domain-induced oligomerization diminished RhoA activation and RhoA-related functions such as cell adhesion and spreading. On the other hand, syndecan-4 phosphorylation-mimicking cytoplasmic domain mutants showed SDS-resistant dimer formation and the similar inhibitory effects as transmembrane domain mutants, but to a less extent, suggesting the existence of domain hierarchy of syndecan-4. Consistently, syndecan-4 transmembrane domain mutant inhibited syndecan-4-mediated protein kinase C activation more efficiently than that of the cytoplasmic domain mutant, and the cytoplasmic domain activation failed to overcome inhibition of transmembrane domain mutants. Taken together, these data suggest that the domain hierarchy of syndecan-4 plays a role in the regulation of syndecan-4 functions.