Syndecans have attached heparan sulfate (HS) and to a lesser extent chondroitin sulfate (CS) chains. These allow interactions with a large number of proteins. Various enzymes involved in post-translational HS chain modifications produce unique binding motifs that selectively recognize different proteins (Tkachenko et al. 2005).

Syndecans are also signalling molecules, interacting with cytoplasmic proteins. Most of the work done has involved syndecan-4 (Multhaupt et al. 2009). Zebrafish and murine syndecan-4 V regions bind the membrane lipid phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) undergoing a shape change revealed by NMR spectroscopy (Oh et al. 1998, Whiteford et al. 2008). The resulting complex is able to bind protein kinase C alpha which is persistently activated in the absence of Ca2+ (Oh et al. 1997, Lee et al. 1998, Keum et al. 2004).