Cell surface proteoglycan that bears heparan sulfate. Regulates exosome biogenesis in concert with SDCBP and PDCD6IP. Homodimer. Interacts with CDCP1 and SDCBP. Interacts (via its cytoplasmic domain) with GIPC (via its PDZ domain). Interacts (via its cytoplasmic domain) with NUDT16L1 (By similarity). Shedding of the ectodomain produces a soluble form. Shedding, cleavage of the extracellular domain to release a soluble form, is enhanced by a number of factors such as heparanase, growth factor receptor action for example by thrombin or EGF. Physiological events such as stress or wound healing can activate the shedding. PMA-mediated shedding is inhibited by TIMP3 (By similarity). Belongs to the syndecan proteoglycan family.