Fig. 1. The three main classes of cell-surface heparan sulfate proteoglycans (HSPGs). (A) Syndecan core proteins are transmembrane proteins that contain a highly conserved C-terminal cytoplasmic domain. Heparan sulfate (HS) chains attach to serine residues distal from the plasma membrane. Some syndecans also contain a chondroitin sulfate (CS) chain(s) that attaches to a serine residue(s) near the membrane. (B) The glypican core proteins are disulphide-stabilized globular core proteins that are linked to the plasma membrane by a glycosylphosphatidylinositol (GPI) linkage. HS chains link to serine residues adjacent to the plasma membrane. (C) Perlecans are secreted HSPGs that carry HS chains.