Cell surface proteoglycan that bears heparan sulfate. Regulates exosome biogenesis in concert with SDCBP and PDCD6IP. Homodimer. Interacts with CDCP1 and SDCBP (By similarity). Interacts (via its cytoplasmic domain) with GIPC (via its PDZ domain). Interacts (via its cytoplasmic domain) with NUDT16L1. Shedding of the ectodomain produces a soluble form. Ubiquitous. Highest levels in liver, kidney and lung. Shedding is enhanced by a number of factors such as heparanase, thrombin or EGF. Also by stress and wound healing. PMA-mediated shedding is inhibited by TIMP3. Belongs to the syndecan proteoglycan family.