Cell surface proteoglycan that may bear heparan sulfate (By similarity). May have a role in the organization of cell shape by affecting the actin cytoskeleton, possibly by transferring signals from the cell surface in a sugar-dependent mechanism. Interacts with TIAM1 (PubMed:23395182). Interacts with PTN (via heparan sulfate chains); this interaction mediates the neurite outgrowth-promoting signal from PTN to the cytoskeleton of growing neurites; this interaction mediates osteoblast recruitment (By similarity). Interacts with MDK; this interaction induces SDC3 clustering; this interaction induces neuronal cell adhesion and neurite outgrowth (By similarity). Expressed in the nervous system, the adrenal gland, and the spleen. O-glycosylated within the Thr/Ser-rich region which could interact with lectin domains on other molecules. Belongs to the syndecan proteoglycan family. Intron retention.