Cell surface proteoglycan that may bear heparan sulfate. 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 (By similarity). Interacts with TIAM1 (By similarity). Interacts (via heparan sulfate chains) with PTN; this interaction mediates the neurite outgrowth-promoting signal from PTN to the cytoskeleton of growing neurites; this interaction mediates osteoblast recruitment (PubMed:8175719, PubMed:9817766). Interacts with MDK; this interaction induces SDC3 clustering; this interaction induces neuronal cell adhesion and neurite outgrowth (PubMed:9089390). High levels in neonatal brain, heart, and Schwann cells, barely detectable in neonatal or adult liver, or adult brain. Higher levels in developing tissues. O-glycosylated within the Thr/Ser-rich region which could interact with lectin domains on other molecules. Belongs to the syndecan proteoglycan family.