Multifunctional adapter protein involved in diverse array of functions including trafficking of transmembrane proteins, neuro and immunomodulation, exosome biogenesis, and tumorigenesis. Positively regulates TGFB1-mediated SMAD2/3 activation and TGFB1-induced epithelial-to-mesenchymal transition (EMT) and cell migration in various cell types. May increase TGFB1 signaling by enhancing cell-surface expression of TGFR1 by preventing the interaction between TGFR1 and CAV1 and subsequent CAV1-dependent internalization and degradation of TGFR1. In concert with SDC1/4 and PDCD6IP, regulates exosome biogenesis. Regulates migration, growth, proliferation, and cell cycle progression in a variety of cancer types. In adherens junctions may function to couple syndecans to cytoskeletal proteins or signaling components. Seems to couple transcription factor SOX4 to the IL-5 receptor (IL5RA). May also play a role in vesicular trafficking. Seems to be required for the targeting of TGFA to the cell surface in the early secretory pathway. Monomer and homodimer (PubMed:11152476). Interacts with NFASC and SDCBP2 (PubMed:11152476). Interacts with PTPRJ (PubMed:11434923). Interacts with SDC1, SDC2, SDC3, SDC4, NRXN2, EPHA7, EPHB1, NF2 isoform 1, TGFA and IL5RA (By similarity). Interacts with PDCD6IP. Forms a complex with PDCD6IP and SDC2. Interacts (via C-terminus) with TGFBR1 (By similarity). Binds to FZD7; this interaction is increased by inositol trisphosphate (IP3) (By similarity). Interacts with SMO (By similarity). Mainly membrane-associated. Localized to adherens junctions, focal adhesions and endoplasmic reticulum. Colocalized with actin stress fibers. Also found in the nucleus. Identified by mass spectrometry in melanosome fractions from stage I to stage IV. Associated to the plasma membrane in the presence of FZD7 and phosphatidylinositol 4,5-bisphosphate (PIP2) (By similarity). Phosphorylated on tyrosine residues.