Cell surface proteoglycan that bears heparan sulfate. Regulates exosome biogenesis in concert with SDCBP and PDCD6IP (PubMed:22660413). Homodimer. Interacts (via its cytoplasmic domain) with GIPC (via its PDZ domain). Interacts (via its cytoplasmic domain) with NUDT16L1 (By similarity). Interacts with CDCP1 and SDCBP. Shedding of the ectodomain produces a soluble form. Expressed in epithelial and fibroblastic cells. 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. Soluble form, lacks the transmembrane domain. Belongs to the syndecan proteoglycan family.